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Here's a super-simple Thursday tip regarding iCal and the Birthdays calendar. The Birthdays calendar is a special one that pulls birthday information from Address Book and displays it as a subscribed calendar in iCal.
At times, though, it seems iCal gets confused, especially if you're syncing via MobileMe. What I've found is that occasionally, and for no apparent reason at all, iCal will create two entries for every birthday. While I realize the importance of not forgetting the birthdays of my contacts, I really don't need to see two reminders of the upcoming event for every single contact.
The temporary fix, at least until someone figures out the bug and fixes it for good, is quite simple. Open iCal's preferences, and on the General tab, uncheck (disable) Show Birthdays Calendar, then check (enable) it again. Presto, no more double birthday entries on the calendar...at least until the next time it happens.
Read article at Macworld.com