User belongs to multiple Groups, same task returened multiple times

we have Users which belong to multiple groups.
eg USER xyz belongs to group sales-department and planning

i create a task and assign both groups as canidates, because both groups can work on this task.
When i display the tasks to the user i call the taskService.findGroupTasks("xyz") and i got the same task 2 times. It depends on to how many groups the user belongs.

I have no idea why this behavior is necessary. I need a workaround for this behavior.

Thanks in advance
Sandro