Information
The mindiff option sets the number of characters in a password that must not be present in the old password.
Use of a complex password helps to increase the time and resources required to compromise the password. Password complexity, or strength, is a measure of the effectiveness of a password in resisting attempts at guessing and brute-force attacks.
Password complexity is one factor of several that determines how long it takes to crack a password. The more complex the password, the greater the number of possible combinations that need to be tested before the password is compromised.
Solution
In /etc/security/user set the default user stanza mindiff attribute to be greater than or equal to 2 :
chsec -f /etc/security/user -s default -a mindiff=2