SOME issue again using CAS Module cann't login in my administration page in application CMS???

Hai all sir , I have a question now I cann't enter my administration page , how to solve that, after I login in my application exchange, I cann't login on it in my application cms

and this is a warning message like this :
user warning: Duplicate entry 'webmaster' for key 2 query: INSERT INTO users (name, pass, init, status, created, access) VALUES ('webmaster', '318b7afe8f5140cec6aa66a6b990c760', 'webmaster', 1, 1276848920, 1276848920) in /var/www/html/content/modules/user/user.module on line 327.

how to solve that?? is there anyone can help me how to back my administration pages???