root@linux:~ # passwd emerson
Changing password for emerson.
Reenter New Password:
Password has been used already. Choose another.
Linux is keeping the old password stored on /etc/security/opasswd. Delete the line containing the user that you’re trying to change the password
You can also check the file /etc/pam.d/common-password and look for a line with the parameter remember.
password required pam_pwhistory.so use_authtok remember=6 retry=3