Multiple birthdays on calendar

I had to disable (and then enable again) sincronization with Exchange Address Book in order to find a soluzion to this issue: https://support.apple.com/it-it/HT209584.

I did it 3 or 4 times. Now I solved that issue but I have multile Birthdays on my calendar!

Every birthday appears 4 times: one refers to the real contact on address book; the three others refer to an empty contact.

It seems that every time I disabled sincronization and deleted the address book all the birthdays weren't erased.

Someone know how to "clean" the birthday calendar?

Naturally I already tried to disable birthday visualization, restart iPhone,....

iPhone XR - iOS 13

Thank you