passwd_duplicate_uid

This built-in function ensures that every account listed in /etc/passwd has a unique UID. This test succeeds if every UID is unique and fails otherwise.

Each user on a Unix system is identified by its User ID (UID), a number comprised between 0 and 65535. If two users share the same UID, then they are not only granted the same privileges, but the system will consider them as being the same person. This defeats any kind of accountability since it is impossible to tell which actions have been performed by each user (typically, the system will do a reverse look up on the UID and will use the first name of the accounts sharing the UID when displaying logs).

Security standards such as the CIS benchmarks forbid sharing a UID among users. If users need to share files, then use groups instead.

Give each user on the system a unique ID.

Usage