addressbook contacts just doubled all entries, and very slow

Hi

On my Laptop, the contacts in the address-books just doubled: Every contact, and every group, is in the database 2 times.

I began to manually merge the doubles, but this is cumbersome: there's 20 years (read: many) contacts, and the address book got very unresponsive, after every action (merging/deleting contacts) I see the spinning pizza for 2,3 seconds, or nothing happens at all for 5+ seconds.

It gets a little bit better if I take the macBook offline.

The last relevant thing I made was reinstalling a big-sur system on a mac mini. I did it the classic way, using the migration assistant, and did not poke around. Just installed it, re-registered apple-id (and very few external things like dropbox), and had it running a very limited time. Didn't touch address book directly.

So I suspect this install could have something to do with sudden double entries?

Could this have a different cause?

Any productive method to solve it?

Any method to diagnose it?

Thank you.

david