Calendar app: birthdays are synced multiple times

I have /e/OS 1.0-20220526188859 on a Samsung S7

each time we load the contacts onto a phone again (this happens when switching to a new /e/os device for example, or reinstalling the OS), a new list of contact birthdays events gets triggered, leading to duplicates.

related community post: https://community.e.foundation/t/birthdays-are-synced-twice-in-built-in-calendar/42282