I've had this Problem for years.. .and have never solved it through all the updates and revision changes in the various upgrade releases in ical. None of the deduping software worked for me, and i couldn't understand why, when for a lot of other people, it did. for me, however, the problem that i discovered is that I had so many different "calendars" that themselves were duplicated, that i had to click on the "calendars" button on the top left of the calendar, and deselect all of the various dupes that had been made after changing from mobile me to the icloud and from my mac to online, etc. etc. so i've chosen to view only the calendars that i wish, and have thus eliminated the duplicates that i was getting. NOW, i can right click on those calendars that i no longer wish to have, and delete them from my computer. Hope this helps anyone who had the same problem as me!