Tasks getting copied over and over

Hi,

we are using Dynamics CRM 2013 on premise. Two weeks ago i created a test Task with the subject "Test Task erstellt im CRM" to verify this task will be synced with my Outlook.

The task has beenn synced with my outlook  client and i deleted the Task in Outlook.

Yesterday i have seen a huge list of the same Task in "My Tasklist". Trying to bulk delete These Tasks does not work. The Tasks are getting copied over and over.

Currently i have 850000 Tasks in my tasklist and it is growing.

What could the Problem be and how to stop the copy of this Task ?

Thanks

Ulrich