Send when Activated Reminder Notifications are sent as long as the Reminder is Active and the Contract is set to Active.
If you make a change in the body of the notification or make no changes and click Save and Close, this generates another notification to users as the Send when Activated option was selected.

If you do not wish to send another notification at that time, make the Reminder Notification inactive or change the Send option to something else, such as 1 day before due, then Save and Close.