You get the following error when trying to view a particular window: "Item has already been added - key indexed (or 'in dictionary'?) 0, Key being added '0'
The issue is caused by either:
Invalid collections, usually on the Group object but can be anywhere
Incorrect tabbing order on the window - turn automatic tabbing off, save, turn it on, test.
Another unknown problem with the window, in which case contact your support provider