Duplicate birthdays in Calendar (iOS 13 public Beta 4)

In the calendar app, the birthday calendar duplicates entries. Now, I'm having already five same birthdays displayed in any calendar app. I've just one entry per person in my contacts app, though. I've tried using Gmail contacts, Apple Contacts and another third-party calendar app. Same issue, still.

Anyone a suggestion how to fix this? Or is this maybe a bug in the beta version of iOS?

Cheers,

Thorsten