iPhone calender creates thousands of duplicate reoccuring entries with exchange 2013

I have a few customers are having issues that their Calender is creating duplicates of reoccuring events, and when I say duplicates I mean thousands of duplicates, and when I delete them, to the point there are none left, it just recreates them. It doesn't recreate them all right awayit will take some time.

After some research I found out that when I disable Calender syncing or delete their Exchange account from their iPhone I can delete the events without them comming back and the bug doesn't not occur anymore. as soon as I enable the calender again it strats creating duplicates again.

This happend with new Iphones, iOS 8 and iOS 9.

There are no problems with other phones or Operating systems.

Has anyone encountered this problem?

I disabled most Calender now, but I am deleting over 10,000 events in small batches, else Outlook crashes. This takes ALLOT of time.

Thanks

Ivar