This is an perfect example of a well designed little utility that does one thing well, without extras or confusion. Anyone who's used synchronization software has run into either one of two situations : You end up with an empty calendar, or you end up with duplicated of everything.
It's been a while since I've had any serious problems with either of these problems (a testament to the maturity of the tools on OS X), but I still have a number of archive calendars that are chock full of duplicate entries from older transactions (mixing google sync, Palm Desktop, Exchange, Entourage and .mac).
I can't remember which web site pointed me to this utility, but it's an absolute must have if you want to clean up your iCal calendars without starting from scratch or passing hours paging through screens and deleting everything that you see.
iCal Dupe Deleter to the rescue. A one trick pony that checks your calendars for duplicates and one click cleans them up for you.
Highly recommended.