Thanks a lot. Disabling the address book has solved the issue of duplicated, non editable contacts for me.
Just another hint to other users having the same issue. The article mentioned by @wwe does not tell you where to put the value ‘system_addressbook_exposed’. Putting it in the nextcloud config.php did not work for me.
Using the occ command worked, but for users that need to use occ web look for this hint to get your occ web app running again for newer versions of nextcloud.