We're having an issue where some people are having duplicate calendar entries appear.  We have a mix of client types (mainly Outlook 2010 for Windows, Mac Mail, and Mac Outlook 2011) on both desktops and laptops, and mobile clients (mainly iPhones, but some blackberries, and some Android).  Only people running Mac Outlook 2011 are seeing this problem.  These people also typically have complex delegate relationships (multiple delegates, additional people with read only access), and a larger number of items in their calendars (most with 2000+ items, some with 5000+ items).  It is affecting many different people, but only one group of 5 consistently.  We are running on Exchange 2010 SP2 RU6.

It may coincide with the user's local cache getting corrupted.  Typically emptying the calendar folder's cache stops the duplication.  However duplicates currently existing continue to be seen.  Occasionally (20% of the time) these duplicates are seen only on Mac Outlook.  Interestingly though - those duplicates are seen on the user's Mac and the delegate's Mac but NOT OWA.  The rest of the time the duplicates are seen on the server and the clients.

We have made sure SyncServices are disabled for all effected users.  iCloud is also disabled.  We've also verified the iTunes is not syncing anything and that phones are only syncing through ActiveSync.  Using Mac Mail Calendar Address Book is not completely viable as contacts are also shared.  Use web only access has been rejected by the users.

Our issue sounds similar to (http://answers.microsoft.com/en-us/mac/forum/macoffice2011-macoutlook/outlook-2011-mailboxes-are-duplicating-many-users/7b18fbb6-71b8-4614-879b-d4480af557f0), however this issue is only effecting Calendar entries.

Has anybody experienced a similar issue?  Anyone have any fixes?

Thanks.