plugin function override problem

I’ve got a super simple plugin that I can’t get working. It basically does one thing. It overrides the wp_get_current_user function (which wordpress docs say can be done. Unfortunately I’m getting the following error when I try and activate my plugin

Fatal error: Cannot redeclare wp_get_current_user() (previously declared in C:\wamp\www\eduweb\mypopstudio.com\mypopstudio.com\blog\wp-includes\pluggable.php:70) in C:\wamp\www\eduweb\mypopstudio.com\mypopstudio.com\blog\wp-content\plugins\mypop_login\mypop_login.php on line 56

Anyone have any ideas?

thanks.