IPhone :: Contacts Multiplied On Office365?

iPhone users within our company are using Office365 for email, contact, calendars.They are facing a very ennoying problem : some contacts are present multiple times (sometime up to 10 times the same contact).We disables suggested contacts in Outlook 2010, upgraded to iOS 5.1.1, removed the account and recreated it. The problem reoccurs always but with different contacts. It seems there's no problem with mail nor calendar. The weired thing is that out of the 10 identical contacts, some are synced with outlook (so we get 2 or 3 times the same contact in Outlook)Also, one of our users says there's no problem with his iPad. What we also see is that is looks like the iPhone deletes all contacts and resyncs (eg: the names in the favorites desappear showing the phone number, then reappear)

Info:
iPhone 4S, iOS 5.1.1