Information
The /etc/security/opasswd file stores the users' old passwords and can be checked to ensure that users are not recycling recent passwords.
Forcing users not to reuse their past 5 passwords make it less likely that an attacker will be able to guess the password.
Note that these change only apply to accounts configured on the local system.
Solution
Set remembered password history to conform to site policy. Many distributions provide tools for updating PAM configuration, consult your documentation for details. If no tooling is provided edit the appropriate /etc/pam.d/ configuration file and add or modify the pam_pwhistory.so or pam_unix.so lines to include the remember option:
password required pam_pwhistory.so remember=5
password required pam_unix.so remember=5