Reminder app - completed items keep notification

My reminder app is frustrating. When I have completed a task, the app keeps notifying me as if the task is not complete. For example, I reminded myself to buy a piece of equipment. I set the length of time to 2 weeks. I completed the task after 1 week, and when the notification pops up, I click COMPLETE but the reminder keeps popping up. Why? To get it to stop I have to enter the app, enter the task and delete the date I had set. That is the only way to make it stop reminding me.

Surely that is not how this app is supposed to work?