If a user account is created, a personal contact is also created for it. If a user account is deleted, the personal contact remains. If the user account is created again later (same login ID), the remaining personal contact is not reused, but a new personal contact is created. This results in duplicate contacts.
As this often occurs in practice, a solution is required.
The remaining personal contact remembers the user ID of their former account, and when a new account is created, the contact is reconnected with the remembered login ID instead of a new contact being created.
Test
In a system with LDAP synchronization, remove accesses from users in the authorization administration and then start the user synchronization (in the scheduler). The accesses to the existing users should then have been restored. No duplicates may be created.