I have SLA on assignment when I submit that assignment at that time task will generate and email will send to manager. But for same task two email notification receive by manager for same correspondence in one day with different time. As per requirement it should send one email notification for one task.
Can you check the SLA configured assignemnt shape. In Advanced Section Notification If it mentioned "NotifyAllAssigne" email might send twice with your specidied SLA time & one day interval time. I had faced this situation once. Please check and confirm.