Information
If the pwhistory enforce_for_root option is enabled, the module will enforce password history for the root user as well
Requiring users not to reuse their passwords make it less likely that an attacker will be able to guess the password or use a compromised password
Note: These change only apply to accounts configured on the local system.
Solution
Edit or add the following line in /etc/security/pwhistory.conf :
enforce_for_root