double exchange birthdays

Problemdescribtion:

Every Exchange-Contact with a birthday generates 2 reccurring appointment-item in the iPhone-iCal.

• There is one reccurring appointment-item generated by Exchange and synchronized with iOS.
• Another appointment is generated by synchronizing the contact with embedded birthday. iOS identifies a birthday and generates a reccurring appointment-item

Please let me know, how can i fix this problem.

Greetings

Fritz