fix todo doublon destinataires notifications

todo/models.py

@@ -81,7 +81,7 @@ class Task(models.Model):
         return get_user_model().objects.filter(
                 ( Q(subscribed_tasklists=self.task_list) & ~Q(unsubscribed_tasks=self)) \
               | (~Q(subscribed_tasklists=self.task_list) &  Q(subscribed_tasks=self))
-        )
+        ).order_by('pk').distinct('pk')
 
     def overdue_status(self):
         "Returns whether the Tasks's due date has passed or not."