Session not destroying properly

Got a problem with sessions. On my index page, there's a place for people to login. Well, when I test with two different user names (ie: mike and jlo) it reverts to the first one when login fails. For example: I enter in the username "myuen" and a wrong password. I get my "Password/Username invalid" message which is what I want. The page refreshes, the username slot already has myuen in it from the last entry, I erase it, enter in a new username "jlo" and the correct password and it will not log me in. I know once you attempt to login, it saves the username as a session variable. On my index.php page, I have a script that checks if a session exists (particularly the Username), if it exists, destroy it otherwise, continue on. Obviously it's not destroying it once we enter in a wrong username and try to login again. Any ideas on how to fix this? Here's my coding: <?PHP if(session_is_registered("CUserName")) session_destroy(); session_register("CUserName"); $CUserNameSession="CUserName"; //SNIP ?> Thanks, Mike