Deduplicate notifications in notification model

bookwyrm/models/notification.py

@@ -25,6 +25,21 @@ class Notification(BookWyrmModel):
     notification_type = models.CharField(
         max_length=255, choices=NotificationType.choices)
 
+    def save(self, *args, **kwargs):
+        ''' save, but don't make dupes '''
+        # there's probably a better way to do this
+        if self.__class__.objects.filter(
+                user=self.user,
+                related_book=self.related_book,
+                related_user=self.related_user,
+                related_status=self.related_status,
+                related_import=self.related_import,
+                related_list_item=self.related_list_item,
+                notification_type=self.notification_type,
+            ).exists():
+            return
+        super().save(*args, **kwargs)
+
     class Meta:
         ''' checks if notifcation is in enum list for valid types '''
         constraints = [