Notifaction.send always creates Celery task with force sending

For sending scheduled notifications in the send_notifications task, Notification.send() is used. If the notification is not sent at the moment, the send_notification task is created with the argument resend=True. Under some circumstances it can happen that send_notifications is executed again before the send_notification tasks were executed. So, more send_notification tasks are created and the user will get the notification more than one time.