iCloud is an amazing feature in OSX Lion. It gives you the ability to sync several devices to the cloud so that your files can be accessible to all those devices without the need to transfer or email those files. However, glitches can happen. It can make apps such as iCal go wonky. Suddenly you have 2 or 3 copies of the same entry and it can get really annoying. Here’s a quick way to get rid of those iCal duplicates you may have. Essentially, just give your iCloud/iCal a reset. Learn here how to remove duplicate iCal entries.
Launch the System Preferences from your Dock. Select the iCloud preference pane located under the Internet and Wireless sector. Once you have the pane open, uncheck and turn off iCloud syncing. Do not worry about the files getting deleted. It will all still be there.
Go back and resync your iCloud account. When prompted, use the same account name and password that you have used before. As iCloud is being reactivated it will prompt you with a question to merge. Click No. Your iCloud already has the files stored and merging will cause it to duplicate itself.
Check all the devices synced with your iCloud. You might have to repeat the same procedure on them.