Seen on 5.1 but current doctrine implementation does not seem to be fixed.

see https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/Persistence/Legacy/User/Gateway/DoctrineDatabase.php

When creating a user, a row is inserted in the ezuser_setting table as well. However, this row is not deleted when deleting the user.

This leads to SQL duplicate key errors when you later try to create a new user which happens to have the same id as a previous user.