If you’re using MobileMe, there are a couple of things to consider before you start trying to clean things up yourself. First, make sure that you’re not syncing with iTunes in addition to MobileMe (which is one of the most common causes of duplicated data). To check this, plug in your iOS device, open iTunes, choose the device from the sidebar on the left, and click on the “Info” tab. If you’re using MobileMe, the iCal and Address Book syncing options within that tab should be toggled off.
iTunes’ data syncing should be off if you’re using MobileMe. Unless you’re a masochist.
The second problem to be aware of is the possibility that the upgrade to the new MobileMe calendar caused duplication of your calendars as a whole (and not individual events). You can resolve this by checking out Apple’s support pages on this issue if you believe this to be the case.
But how do you manually fix duplicated entries? Address Book makes this fairly simple with its built-in Card > Look for Duplicates menu item. If you run that, you’ll walk through the process of putting things right, and I’ve found that this works pretty darned well.
If you end up having to do some more cleanup after it runs, an easy way to do so is to select multiple cards that you’d like to merge (by clicking one, holding down the Command key, and then clicking the others). You can then choose Card > Merge Selected Cards, and all of your existing information within those cards will be retained in just one.
What of iCal, though? It doesn’t offer a built-in way to resolve duplicated events, and doing it by hand can make you want to go full Office Space on your computer. Luckily, a wonderful individual named John Maisey has created the donationware program iCal Dupe Deleter. I’ve found this program to be incredibly useful on a number of occasions, as I’m not excited by the prospect of spending a Saturday afternoon removing 1,063 duplicated events informing me that the Rockies are playing at 6:00.
As always, you should make sure that you back up your existing calendars and Address Book before you make any changes (and I’ve linked here to the Apple support information on how to do so). It’d be a bummer to try and resolve your duplicates only to find out that you’ve lost everything you were trying to consolidate!
They mostly come at night. Mostly.