Stop multiplying birthday calendars in iCal?

It seems that every once in a while I notice that iCal (or address book, I don't know) creates a new and redundant birthday calendar in iCal. I often don't notice it until there are 4 or 5 birthday calendars, so that every birthday reminder is repeated 4 or 5 times. I have iCal set to generate birthday reminders from my address book records, but I don't see why it would keep multiplying the reminders.

Any suggestions?

Thanks!