If an account other than root also has a User Identifier (UID) of "0", it has root authority, giving that account unrestricted access to the entire operating system. Multiple accounts with a UID of "0" afford an opportunity for potential intruders to guess a password for a privileged account.
Check
Check the system for duplicate UID "0" assignments with the following command:
$ sudo awk -F: '$3 == 0 {print $1}' /etc/passwd
If any accounts other than root have a UID of "0", this is a finding.
Fix
Change the UID of any account on the system, other than root, that has a UID of "0".
If the account is associated with system commands or applications, the UID should be changed to one greater than "0" but less than "1000". Otherwise, assign a UID of greater than "1000" that has not already been assigned.