Have you entered them manually into the Calendar app or do you have the Birthdays calendar ticked so they are picked up from the Contacts app? Maybe both?
if they're picked up from Contacts, it suggests you have duplicate (or triplicate!) contacts. Check that first for one of the offending birthdays. (If you have slightly different data on each card, there is a "merge cards" option.)
Unticking the birthdays calendar will only hide it - ticking it will bring it back.
Michael Corgan 