Event duplication in iCloud accounts
I have had a couple of emails in the last few days in relation to Apple's iCloud and duplicates in iCal. Having used iCal Dupe Deleter the duplicates for some users are re-appearing in iCal after a short while. The addition of the duplicated events is due to iCloud re-introducing the duplicated events back into the calendars from the server.
Apple have created a knowledge base article (TS4038) that outlines how they suggest users to remove duplicates in MobileMe. This method is identical to that for iCloud calendars and (although it seems long winded) I suggest you follow these instructions when de-duping events from iCloud calendars.