Check that user and group names are unique

[adombeck]

Unix user and group names must be unique on the system. For example, /etc/group assigns users to groups by name, not by UID. Duplicate group names would lead to unexpected behavior of getgrnam and tools like chown.

We currently don't check if a user name is used by any other user on the system (that's not in our database).

For group names, we could even have multiple groups with different GIDs but same names in our database, as the test case added in 7017636 demonstrates.