The list is located in the /etc/security/opasswd file. This is not a plain text file, but it should be protected the same as the /etc/shadow file. This is normally referred to as password history.
To remember the last 5 passwords, add the line below to the /etc/pam.d/system-auth file:
password sufficient /lib/security/pam_unix.so use_authtok md5 shadow remember=5
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source:http://linuxpoison.blogspot.com/2007/11/13578175802121.html