Luckily, John Maisey has created both Leopard and Tiger versions of an AppleScript that will sift through the calendar of your choice and kill off duplicate appointments. Don't be alarmed if it doesn't appear to be doing anything when you first run it. In our brief test, it took about 13 seconds to delete 15 duplicate entries, so this script is no speed demon. But what it lacks in terms of speed, it more than makes up in utility.
As with any script or utility that deletes data, be smart and back up your iCal data before you try it.