When syncing contacts that I just created or edited on my laptop they immediatly multiply many times, up to 100.
When I delete the duplicates, they reappear next time, until after a couple of times they become fewer and then it stops.
They do stay deleted immediatly if I login to my mail provider's website and delete them there.
The second device synced with this account is an Android 7.1 phone with the CardDav-Sync free app.
It doesn't seem to happen when I create or edit contacts on the phone.
This has been happening for a couple of months now.
The mail provider's support says the problem must be with this addon.