Have just noticed that birthdays in iCal on my Mac are duplicated. Deselecting 'Birthdays' (Subscription calendar) in iCal removes ALL occurences of birthdays. The iCloud.com #calendar shows no duplicate and all birthdays seem to be on display; however, selective birthdays are not appearing in Calendar on my iPhone 4S (but, otherwise, there are no duplicates!).
Anyone any ideas on what has gone wrong? The fault(s) are certainly recent, as I know all was in order on Friday last (24/2/2012).
Look in your address book (on your mac), do you have iCloud as well as On My Mac accounts? if you do that is where the dupes are coming from.