Passwords for existing users must have a 24-hour minimum password lifetime restriction in /etc/shadow.

Enforcing a minimum password lifetime helps to prevent repeated password changes to defeat the password reuse or history enforcement requirement. If users are allowed to immediately and continually change their password, then the password could be repeatedly changed in a short period of time to defeat the organization's policy regarding password reuse. Checking existing users have this setting will ensure that no users created before the policy was configured can evade the password minimum setting.