There is a known issue with Entourage where users may see multiple copies of the same items in their mailbox. The most frequent reports are on Calendar items or Contacts being duplicated, triplicated, or often times even more copies appear. This can happen when a user has a mobile device set to synchronize "over the air" directly with Exchange and is using the Sync Services in Entourage (which syncs mailbox data through Apple’s Sync Engine). Disabling the use of Sync Services in Entourage may resolve the issue with duplication.
To disable the Sync Services in Entourage follow these steps:
1. Open Entourage and go to Preferences > General Preferences > Sync Services.
2. Uncheck all 3 boxes:
- Synchronize contacts with the Address Book and .Mac
- Synchronize events and tasks with iCal and .Mac
- Synchronize Entourage Notes with .Mac
3. Restart Entourage for the changes to take effect.
If you still see multiple copies of the same item, you may refer to the article How Do I Fix Entourage And Outlook 2011 Synchronization Issues?