birthday duplicates after removing google account and adding it again

After removing my Google-Account on my HTC One S and adding it again, i recognized that all birthdays stored with contacts are in my calendar twice. When i change birthday at some contact, there is only one, which changes. The information is not from another calendar like Facebook or Google. After removing my account once and adding it, i had two birthdays for each contact, only one changes. After removing my account another time and adding it, i had three birthdays for each conatct and only one changes. So why this happens or how can i delete or avoid this? I would like the delete either automically or manually (prefering automatically) but i found no solution to do this. I made a screenshot to make it clearer.