I've had several instances of birthdays in the iCal birthday calendar showing up twice. It finally bugged me enough to look at. I had Syncrospector up and was about to perform surgery, when I tried something simpler that didn't require a scalpel.
Uncheck Show Birthdays calendar in iCal's preferences, then recheck it. That cleaned it out for me.
[robg adds: You can read more about Syncrospector in this hint, or in this tutorial on Apple's site.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081112074836249