After adding a few devices to iCloud, iCal is showing duplicates and triplicates for events going back to when I started using iCal.
This is not due to:
a) Having multiple accounts: I have just one iCloud account
b) Having different types of calendars: All events are on iCloud only
c) Syncing in iTunes is turned off for all devices (automatic for iCloud)
The duplicates are visible when I log into iCloud.com which represents "the truth". The duplicates are also visible from my iOS devices and computers, so it's consistent across the board. iCal and iCloud appear to be working normally when new events are added.
1. Disconnecting/reconnecting from iCloud.
2. iCal Dupe Deleter:
This is an awesome tool but the blog post above suggests that iCloud will respawn the duplicates.
3. Deleting events manually: I'm willing to take the brute force approach but there is a further complication: When I try to delete events that are attached to other people (who invited me or vice versa) iCal will prompt me to say it will decline the invitation. There used to be a way to simply not send a notification but now you are required to when you delete an invite. There is another handy tool by the same fellow called "iCal Reply Checker" which prevents Mail from spamming people, however it doesn't seem to be able to stop iCal from talking to iCloud which will send the decline notice on your behalf.
Bonus irritation: When I added my devices, iCloud took the opportunity to send dozens of notices to work associates saying that I had accepted their meeting requests from years ago. It did this without asking.