feat(notifications): deduplicate repeated email alerts

Add notification-level deduplication keys and TTLs for deployment, backup,
server, container, scheduled task, SSL, token, and transactional emails.
Apply deduplication in email channels before sending rendered messages.

app/Notifications/ApiTokenExpiringNotification.php

@@ -29,6 +29,16 @@ class ApiTokenExpiringNotification extends CustomEmailNotification
         return $notifiable->getEnabledChannels('api_token_expiring');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "api-token-expiring:{$this->token->id}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 172800;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Application/DeploymentFailed.php

@@ -52,6 +52,16 @@ class DeploymentFailed extends CustomEmailNotification
         return $notifiable->getEnabledChannels('deployment_failure');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "deployment-failed:{$this->deployment_uuid}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Application/DeploymentSuccess.php

@@ -52,6 +52,16 @@ class DeploymentSuccess extends CustomEmailNotification
         return $notifiable->getEnabledChannels('deployment_success');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "deployment-success:{$this->deployment_uuid}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Application/RestartLimitReached.php

@@ -49,6 +49,16 @@ class RestartLimitReached extends CustomEmailNotification
         return $notifiable->getEnabledChannels('status_change');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "restart-limit-reached:application:{$this->resource->uuid}:count:{$this->restart_count}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Application/StatusChanged.php

@@ -42,6 +42,16 @@ class StatusChanged extends CustomEmailNotification
         return $notifiable->getEnabledChannels('status_change');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "application-status-changed:application:{$this->resource->uuid}:stopped";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 3600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;
@@ -50,6 +60,7 @@ class StatusChanged extends CustomEmailNotification
         $mail->view('emails.application-status-changes', [
             'name' => $this->resource_name,
             'fqdn' => $fqdn,
+            'application_url' => $this->resource_url,
             'resource_url' => $this->resource_url,
         ]);
 

app/Notifications/Channels/EmailChannel.php

@@ -4,13 +4,20 @@ namespace App\Notifications\Channels;
 
 use App\Exceptions\NonReportableException;
 use App\Models\Team;
+use App\Services\NotificationDeduplicator;
 use Exception;
 use Illuminate\Notifications\Notification;
 use Resend;
+use Resend\Exceptions\ErrorException;
+use Resend\Exceptions\TransporterException;
+use Symfony\Component\Mailer\Mailer;
+use Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport;
+use Symfony\Component\Mime\Address;
+use Symfony\Component\Mime\Email;
 
 class EmailChannel
 {
-    public function __construct() {}
+    public function __construct(private NotificationDeduplicator $deduplicator) {}
 
     public function send(SendsEmail $notifiable, Notification $notification): void
     {
@@ -67,6 +74,11 @@ class EmailChannel
             }
 
             $mailMessage = $notification->toMail($notifiable);
+            $renderedMail = (string) $mailMessage->render();
+
+            if (! $this->deduplicator->shouldSend($notifiable, $notification, self::class, $recipients, $mailMessage->subject, $renderedMail)) {
+                return;
+            }
 
             if ($isResendEnabled) {
                 $resend = Resend::client($settings->resend_api_key);
@@ -75,17 +87,17 @@ class EmailChannel
                     'from' => $from,
                     'to' => $recipients,
                     'subject' => $mailMessage->subject,
-                    'html' => (string) $mailMessage->render(),
+                    'html' => $renderedMail,
                 ]);
             } elseif ($isSmtpEnabled) {
-                $encryption = match (strtolower($settings->smtp_encryption)) {
+                $encryption = match (strtolower($settings->smtp_encryption ?? '')) {
                     'starttls' => null,
                     'tls' => 'tls',
                     'none' => null,
                     default => null,
                 };
 
-                $transport = new \Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport(
+                $transport = new EsmtpTransport(
                     $settings->smtp_host,
                     $settings->smtp_port,
                     $encryption
@@ -93,20 +105,20 @@ class EmailChannel
                 $transport->setUsername($settings->smtp_username ?? '');
                 $transport->setPassword($settings->smtp_password ?? '');
 
-                $mailer = new \Symfony\Component\Mailer\Mailer($transport);
+                $mailer = new Mailer($transport);
 
                 $fromEmail = $settings->smtp_from_address ?? 'noreply@localhost';
                 $fromName = $settings->smtp_from_name ?? 'System';
-                $from = new \Symfony\Component\Mime\Address($fromEmail, $fromName);
-                $email = (new \Symfony\Component\Mime\Email)
+                $from = new Address($fromEmail, $fromName);
+                $email = (new Email)
                     ->from($from)
                     ->to(...$recipients)
                     ->subject($mailMessage->subject)
-                    ->html((string) $mailMessage->render());
+                    ->html($renderedMail);
 
                 $mailer->send($email);
             }
-        } catch (\Resend\Exceptions\ErrorException $e) {
+        } catch (ErrorException $e) {
             // Map HTTP status codes to user-friendly messages
             $userMessage = match ($e->getErrorCode()) {
                 403 => 'Invalid Resend API key. Please verify your API key in the Resend dashboard and update it in settings.',
@@ -131,13 +143,13 @@ class EmailChannel
 
             // Don't report expected errors (invalid keys, validation) to Sentry
             if (in_array($e->getErrorCode(), [403, 401, 400])) {
-                throw NonReportableException::fromException(new \Exception($userMessage, $e->getCode(), $e));
+                throw NonReportableException::fromException(new Exception($userMessage, $e->getCode(), $e));
             }
 
-            throw new \Exception($userMessage, $e->getCode(), $e);
-        } catch (\Resend\Exceptions\TransporterException $e) {
+            throw new Exception($userMessage, $e->getCode(), $e);
+        } catch (TransporterException $e) {
             send_internal_notification("Resend Transport Error: {$e->getMessage()}");
-            throw new \Exception('Unable to connect to Resend API. Please check your internet connection and try again.');
+            throw new Exception('Unable to connect to Resend API. Please check your internet connection and try again.');
         } catch (\Throwable $e) {
             // Check if this is a Resend domain verification error on cloud instances
             if (isCloud() && str_contains($e->getMessage(), 'domain is not verified')) {

app/Notifications/Channels/TransactionalEmailChannel.php

@@ -3,6 +3,7 @@
 namespace App\Notifications\Channels;
 
 use App\Models\User;
+use App\Services\NotificationDeduplicator;
 use Exception;
 use Illuminate\Mail\Message;
 use Illuminate\Notifications\Notification;
@@ -10,6 +11,8 @@ use Illuminate\Support\Facades\Mail;
 
 class TransactionalEmailChannel
 {
+    public function __construct(private NotificationDeduplicator $deduplicator) {}
+
     public function send(User $notifiable, Notification $notification): void
     {
         $settings = instanceSettings();
@@ -27,13 +30,19 @@ class TransactionalEmailChannel
         }
         $this->bootConfigs();
         $mailMessage = $notification->toMail($notifiable);
+        $renderedMail = (string) $mailMessage->render();
+
+        if (! $this->deduplicator->shouldSend($notifiable, $notification, self::class, [$email], $mailMessage->subject, $renderedMail)) {
+            return;
+        }
+
         Mail::send(
             [],
             [],
             fn (Message $message) => $message
                 ->to($email)
                 ->subject($mailMessage->subject)
-                ->html((string) $mailMessage->render())
+                ->html($renderedMail)
         );
     }
 

app/Notifications/Container/ContainerRestarted.php

@@ -21,6 +21,16 @@ class ContainerRestarted extends CustomEmailNotification
         return $notifiable->getEnabledChannels('status_change');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "container-restarted:server:{$this->server->uuid}:container:{$this->name}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 3600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Container/ContainerStopped.php

@@ -21,6 +21,16 @@ class ContainerStopped extends CustomEmailNotification
         return $notifiable->getEnabledChannels('status_change');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "container-stopped:server:{$this->server->uuid}:container:{$this->name}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 3600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/CustomEmailNotification.php

@@ -15,4 +15,19 @@ class CustomEmailNotification extends Notification implements ShouldQueue
     public $tries = 5;
 
     public $maxExceptions = 5;
+
+    public function shouldDeduplicate(): bool
+    {
+        return true;
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 900;
+    }
+
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return null;
+    }
 }

app/Notifications/Database/BackupFailed.php

@@ -11,6 +11,8 @@ use Illuminate\Notifications\Messages\MailMessage;
 
 class BackupFailed extends CustomEmailNotification
 {
+    public int|string|null $backupId = null;
+
     public string $name;
 
     public string $frequency;
@@ -18,6 +20,7 @@ class BackupFailed extends CustomEmailNotification
     public function __construct(ScheduledDatabaseBackup $backup, public $database, public $output, public $database_name)
     {
         $this->onQueue('high');
+        $this->backupId = data_get($backup, 'uuid') ?? data_get($backup, 'id');
         $this->name = $database->name;
         $this->frequency = $backup->frequency;
     }
@@ -27,6 +30,16 @@ class BackupFailed extends CustomEmailNotification
         return $notifiable->getEnabledChannels('backup_failure');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "backup-failed:backup:{$this->backupId}:database:{$this->database->uuid}:output:".hash('sha256', (string) $this->output);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 21600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Database/BackupSuccess.php

@@ -11,6 +11,8 @@ use Illuminate\Notifications\Messages\MailMessage;
 
 class BackupSuccess extends CustomEmailNotification
 {
+    public int|string|null $backupId = null;
+
     public string $name;
 
     public string $frequency;
@@ -18,6 +20,7 @@ class BackupSuccess extends CustomEmailNotification
     public function __construct(ScheduledDatabaseBackup $backup, public $database, public $database_name)
     {
         $this->onQueue('high');
+        $this->backupId = data_get($backup, 'uuid') ?? data_get($backup, 'id');
 
         $this->name = $database->name;
         $this->frequency = $backup->frequency;
@@ -28,6 +31,16 @@ class BackupSuccess extends CustomEmailNotification
         return $notifiable->getEnabledChannels('backup_success');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "backup-success:backup:{$this->backupId}:database:{$this->database->uuid}:name:{$this->database_name}:frequency:{$this->frequency}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Database/BackupSuccessWithS3Warning.php

@@ -11,6 +11,8 @@ use Illuminate\Notifications\Messages\MailMessage;
 
 class BackupSuccessWithS3Warning extends CustomEmailNotification
 {
+    public int|string|null $backupId = null;
+
     public string $name;
 
     public string $frequency;
@@ -20,6 +22,7 @@ class BackupSuccessWithS3Warning extends CustomEmailNotification
     public function __construct(ScheduledDatabaseBackup $backup, public $database, public $database_name, public $s3_error)
     {
         $this->onQueue('high');
+        $this->backupId = data_get($backup, 'uuid') ?? data_get($backup, 'id');
 
         $this->name = $database->name;
         $this->frequency = $backup->frequency;
@@ -34,6 +37,16 @@ class BackupSuccessWithS3Warning extends CustomEmailNotification
         return $notifiable->getEnabledChannels('backup_failure');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "backup-s3-warning:backup:{$this->backupId}:database:{$this->database->uuid}:error:".hash('sha256', (string) $this->s3_error);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 21600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/ScheduledTask/TaskFailed.php

@@ -28,6 +28,16 @@ class TaskFailed extends CustomEmailNotification
         return $notifiable->getEnabledChannels('scheduled_task_failure');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "scheduled-task-failed:task:{$this->task->uuid}:output:".hash('sha256', $this->output);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 3600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/ScheduledTask/TaskSuccess.php

@@ -28,6 +28,16 @@ class TaskSuccess extends CustomEmailNotification
         return $notifiable->getEnabledChannels('scheduled_task_success');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "scheduled-task-success:task:{$this->task->uuid}:output:".hash('sha256', $this->output);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 3600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/DockerCleanupFailed.php

@@ -21,6 +21,16 @@ class DockerCleanupFailed extends CustomEmailNotification
         return $notifiable->getEnabledChannels('docker_cleanup_failure');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "docker-cleanup-failed:server:{$this->server->uuid}:message:".hash('sha256', $this->message);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 21600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/DockerCleanupSuccess.php

@@ -21,6 +21,16 @@ class DockerCleanupSuccess extends CustomEmailNotification
         return $notifiable->getEnabledChannels('docker_cleanup_success');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "docker-cleanup-success:server:{$this->server->uuid}:message:".hash('sha256', $this->message);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 21600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/ForceDisabled.php

@@ -21,6 +21,16 @@ class ForceDisabled extends CustomEmailNotification
         return $notifiable->getEnabledChannels('server_force_disabled');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "server-force-disabled:{$this->server->uuid}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/ForceEnabled.php

@@ -21,6 +21,16 @@ class ForceEnabled extends CustomEmailNotification
         return $notifiable->getEnabledChannels('server_force_enabled');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "server-force-enabled:{$this->server->uuid}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/HetznerDeletionFailed.php

@@ -21,6 +21,16 @@ class HetznerDeletionFailed extends CustomEmailNotification
         return $notifiable->getEnabledChannels('hetzner_deletion_failed');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "hetzner-deletion-failed:{$this->hetznerServerId}:error:".hash('sha256', $this->errorMessage);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/HighDiskUsage.php

@@ -21,6 +21,16 @@ class HighDiskUsage extends CustomEmailNotification
         return $notifiable->getEnabledChannels('server_disk_usage');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "high-disk-usage:server:{$this->server->uuid}:threshold:{$this->server_disk_usage_notification_threshold}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 21600;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/Reachable.php

@@ -30,6 +30,16 @@ class Reachable extends CustomEmailNotification
         return $notifiable->getEnabledChannels('server_reachable');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "server-reachable:{$this->server->uuid}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 1800;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/ServerPatchCheck.php

@@ -24,6 +24,16 @@ class ServerPatchCheck extends CustomEmailNotification
         return $notifiable->getEnabledChannels('server_patch');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "server-patch-check:server:{$this->server->uuid}:state:".hash('sha256', json_encode($this->patchData));
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail($notifiable = null): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/TraefikVersionOutdated.php

@@ -38,6 +38,18 @@ class TraefikVersionOutdated extends CustomEmailNotification
         return $this->formatVersion($info['latest'] ?? 'unknown');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        $serverUuids = $this->servers->pluck('uuid')->sort()->values()->join('|');
+
+        return 'traefik-version-outdated:servers:'.hash('sha256', $serverUuids);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail($notifiable = null): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Server/Unreachable.php

@@ -30,6 +30,16 @@ class Unreachable extends CustomEmailNotification
         return $notifiable->getEnabledChannels('server_unreachable');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "server-unreachable:{$this->server->uuid}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 3600;
+    }
+
     public function toMail(): ?MailMessage
     {
         $mail = new MailMessage;

app/Notifications/SslExpirationNotification.php

@@ -59,6 +59,22 @@ class SslExpirationNotification extends CustomEmailNotification
         return $notifiable->getEnabledChannels('ssl_certificate_renewal');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        $resourceKeys = $this->resources
+            ->map(fn ($resource) => data_get($resource, 'uuid') ?? data_get($resource, 'name'))
+            ->sort()
+            ->values()
+            ->join('|');
+
+        return 'ssl-certificate-renewed:resources:'.hash('sha256', $resourceKeys);
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail(): MailMessage
     {
         $mail = new MailMessage;

app/Notifications/Test.php

@@ -30,6 +30,11 @@ class Test extends Notification implements ShouldQueue
         $this->onQueue('high');
     }
 
+    public function shouldDeduplicate(): bool
+    {
+        return false;
+    }
+
     public function via(object $notifiable): array
     {
         if ($this->channel) {

app/Notifications/TransactionalEmails/EmailChangeVerification.php

@@ -25,6 +25,16 @@ class EmailChangeVerification extends CustomEmailNotification
         $this->onQueue('high');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "email-change-verification:user:{$this->user->id}:email:{$this->newEmail}:code:{$this->verificationCode}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return (int) max(1, now()->diffInSeconds($this->expiresAt, false));
+    }
+
     public function toMail(): MailMessage
     {
         // Use the configured expiry minutes value

app/Notifications/TransactionalEmails/InvitationLink.php

@@ -21,6 +21,16 @@ class InvitationLink extends CustomEmailNotification
         $this->onQueue('high');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "invitation-link:user:{$this->user->id}:email:{$this->user->email}";
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 3600;
+    }
+
     public function toMail(): MailMessage
     {
         $invitation = TeamInvitation::whereEmail($this->user->email)->first();

app/Notifications/TransactionalEmails/Test.php

@@ -15,6 +15,11 @@ class Test extends CustomEmailNotification
         $this->onQueue('high');
     }
 
+    public function shouldDeduplicate(): bool
+    {
+        return false;
+    }
+
     public function via(): array
     {
         return [EmailChannel::class];

app/Services/NotificationDeduplicator.php

@@ -0,0 +1,95 @@
+<?php
+
+namespace App\Services;
+
+use Illuminate\Notifications\Notification;
+use Illuminate\Support\Facades\Cache;
+
+class NotificationDeduplicator
+{
+    public const DEFAULT_TTL = 900;
+
+    /**
+     * @param  array<int, string>  $recipients
+     */
+    public function shouldSend(object $notifiable, Notification $notification, string $channel, array $recipients, ?string $subject = null, ?string $body = null): bool
+    {
+        if (method_exists($notification, 'shouldDeduplicate') && ! $notification->shouldDeduplicate()) {
+            return true;
+        }
+
+        $ttl = method_exists($notification, 'deduplicateFor')
+            ? $notification->deduplicateFor()
+            : self::DEFAULT_TTL;
+
+        if ($ttl <= 0) {
+            return true;
+        }
+
+        return Cache::add(
+            $this->cacheKey($notifiable, $notification, $channel, $recipients, $subject, $body),
+            true,
+            $ttl,
+        );
+    }
+
+    /**
+     * @param  array<int, string>  $recipients
+     */
+    private function cacheKey(object $notifiable, Notification $notification, string $channel, array $recipients, ?string $subject, ?string $body): string
+    {
+        $semanticKey = method_exists($notification, 'deduplicationKey')
+            ? $notification->deduplicationKey($notifiable, $channel)
+            : null;
+
+        $payload = $semanticKey
+            ? $this->semanticFingerprint($notifiable, $notification, $channel, $recipients, $semanticKey)
+            : $this->defaultFingerprint($notifiable, $notification, $channel, $recipients, $subject, $body);
+
+        return 'notification-dedupe:'.hash('sha256', $payload);
+    }
+
+    /**
+     * @param  array<int, string>  $recipients
+     */
+    private function semanticFingerprint(object $notifiable, Notification $notification, string $channel, array $recipients, string $semanticKey): string
+    {
+        return json_encode([
+            'notification' => $notification::class,
+            'notifiable' => $notifiable::class,
+            'notifiable_id' => data_get($notifiable, 'id'),
+            'channel' => $channel,
+            'recipients' => $this->normalizeRecipients($recipients),
+            'semantic_key' => $semanticKey,
+        ], JSON_THROW_ON_ERROR);
+    }
+
+    /**
+     * @param  array<int, string>  $recipients
+     */
+    private function defaultFingerprint(object $notifiable, Notification $notification, string $channel, array $recipients, ?string $subject, ?string $body): string
+    {
+        return json_encode([
+            'notification' => $notification::class,
+            'notifiable' => $notifiable::class,
+            'notifiable_id' => data_get($notifiable, 'id'),
+            'channel' => $channel,
+            'recipients' => $this->normalizeRecipients($recipients),
+            'subject' => $subject,
+            'body_hash' => hash('sha256', (string) $body),
+        ], JSON_THROW_ON_ERROR);
+    }
+
+    /**
+     * @param  array<int, string>  $recipients
+     * @return array<int, string>
+     */
+    private function normalizeRecipients(array $recipients): array
+    {
+        return collect($recipients)
+            ->map(fn (string $recipient) => mb_strtolower(trim($recipient)))
+            ->sort()
+            ->values()
+            ->all();
+    }
+}

tests/Feature/ApplicationStoppedAfterRestartLimitTest.php

@@ -48,6 +48,24 @@ it('does not show the restart limit warning for a normal manual stop', function
     expect($html)->not->toContain('Stopped after reaching restart limit');
 });
 
+it('uses a semantic dedupe key for restart limit notifications', function () {
+    $application = applicationWithRestartState();
+    $application->forceFill([
+        'name' => 'crashy-app',
+        'uuid' => 'application-uuid',
+    ]);
+    $application->setRelation('environment', (object) [
+        'uuid' => 'environment-uuid',
+        'name' => 'production',
+        'project' => (object) ['uuid' => 'project-uuid'],
+    ]);
+
+    $notification = new RestartLimitReached($application);
+
+    expect($notification->deduplicationKey((object) ['id' => 1], 'mail'))->toBe('restart-limit-reached:application:application-uuid:count:2')
+        ->and($notification->deduplicateFor())->toBe(86400);
+});
+
 it('keeps restart tracking configurable when stopping an application', function () {
     $method = new ReflectionMethod(StopApplication::class, 'handle');
     $resetRestartCount = collect($method->getParameters())->firstWhere('name', 'resetRestartCount');

tests/Feature/NotificationDeduplicationTest.php

@@ -0,0 +1,75 @@
+<?php
+
+use App\Notifications\CustomEmailNotification;
+use App\Services\NotificationDeduplicator;
+use Illuminate\Notifications\Messages\MailMessage;
+use Illuminate\Support\Facades\Cache;
+
+class DedupeTestNotification extends CustomEmailNotification
+{
+    public function __construct(
+        public ?string $semanticKey = null,
+        public bool $deduplicate = true,
+        public int $ttl = 900,
+    ) {}
+
+    public function toMail(): MailMessage
+    {
+        return (new MailMessage)->subject('Test');
+    }
+
+    public function shouldDeduplicate(): bool
+    {
+        return $this->deduplicate;
+    }
+
+    public function deduplicateFor(): int
+    {
+        return $this->ttl;
+    }
+
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return $this->semanticKey;
+    }
+}
+
+beforeEach(function () {
+    Cache::flush();
+    $this->deduplicator = app(NotificationDeduplicator::class);
+    $this->notifiable = new class
+    {
+        public int $id = 123;
+    };
+});
+
+it('allows only the first identical notification fingerprint during the ttl', function () {
+    $notification = new DedupeTestNotification;
+
+    expect($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeFalse();
+});
+
+it('allows different recipients and content through the default fingerprint', function () {
+    $notification = new DedupeTestNotification;
+
+    expect($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['second@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Other subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Other body</p>'))->toBeTrue();
+});
+
+it('uses semantic keys instead of rendered content when provided', function () {
+    $notification = new DedupeTestNotification(semanticKey: 'event:123');
+
+    expect($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Other subject', '<p>Other body</p>'))->toBeFalse()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['second@example.com'], 'Other subject', '<p>Other body</p>'))->toBeTrue();
+});
+
+it('allows notifications to opt out of deduplication', function () {
+    $notification = new DedupeTestNotification(deduplicate: false);
+
+    expect($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue();
+});