This is the single biggest problem I’ve found with Entourage to iCal synchronization. Entourage sync is incredibly slow on Leopard to the calendar, but more importantly, with the iPhone hooked in, you get zillions of duplicates.
The Mac Address Book has a built in deduplicator which sort of works, but is better than nothing. With iCal, you go to “iCal Dupe deleter”:http://www.nhoj.co.uk/icaldupedeleter/ which is a nice little application someone wrote to clean up iCal. I’m in the middle of deleting 2500 duplicate events right now!