Duplicate birthdays in Calendar but only on Mac

I have my birthdays stored in my contacts. As of late I see duplicate entries for each birthday in my Calendar on the Mac. On my iOS devices they only show up once. When I deactivate my contacts account (macOS Server account) the birthdays are removed from my calendar as expected. When I reactivate the contacts account the birthdays re-appear but only once. After a few hours Calendar on my Mac will then show them twice again. If I try to manually delete one of the events I get the error message: "You can't add or change events in the Birthdays calendar". Is there a why for me to reset my "birthdays Calendar" on my Mac? What else can I do? I am using the Sierra 10.12.2 and Calendar version 9.0 Thanks for any advice.