Start by checking if the duplicate events belong to the same calendar. If not, then you can delete one of the calendars. I've had this happen in the past with MobileMe, where a calendar was duplicated and I ended up with one local version and one MobileMe version.
How many Macs and iDevices are in your sync loop? Also, make sure that you don't set up sync in two places simultaneously, that iCal sync is disabled in iTunes if you use MobileMe or iCloud.