Not sure what steps cause this problem to happen, but I've had iCal freak out (on Leopard) where it starts showing all of the birthdays in the calendar twice. What worked for me is going into the iCal preferences and in the "General" tab, unchecking the box that says "Show Birthdays calendar". Then close the preferences. Then go back to Preferences again and recheck this box. When I did this the birthdays were then just shown once in iCal.