/etc/pam.d/passwd
#%PAM-1.0
auth required pam_stack.so service=system-auth
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
password requisite pam_cracklib.so dcredit=-1 ucredit=-1 lcredit=-1 ocredit=-1 minlen=6
password required pam_unix.so use_authtok nullok md5
password required pam_unix.so remember=5 use_authtok

it needs input four times to change password
like this：
[root@localhost pam.d]# passwd gkpt
Changing password for user gkpt.
New UNIX password:
Retype new UNIX password:
New UNIX password:


when i delete the fourth line “password required pam_stack.so service=system-auth”，i can change passwd successfully！

is this line conflict with others？thanks！

0) Unless this is an exotic or ancient system you shouldn't use pam_stack as pass through but "include system-auth" lines.
1) The password cracklib and pam_unix lines should be in /etc/pam.d/system-auth-ac.
2) You have a duplicate "password pam_unix" line.
3) Check /var/log/secure for clues.