iCal birthdays showing up twice?

Hello!

I have a problem where all birthdays on my Mac are showing up twice. This problem has occured only now. I've had iCloud and Mountain Lion since they came out. And I am using iOS 5 on my iPhone. The birthdays don't show up twice on my iPhone, only on my Mac in iCal. Also, not all of them show up twice. Only those in the future.

I don't synchronize my calendar or my contacts or anything of this kind through iTunes, I use iCloud for that. So how come they show up twice?

I hope someone can help me! I'd really appreciate that 🙂

Greetings