Begin on "Whats wrong here?"

Trying to make the simplest plugin to disable the admin bar and get error:

Fatal error: Cannot redeclare remove_admin_bar() (previously declared in /home/veinspec/Home/MySites/Tests/Man2/wp-content/themes/twentytwelve/functions.php:507) in /home/veinspec/Home/MySites/Tests/Man2/wp-content/plugins/DisableAdminBar/DisableAdminBar.php on line 11

This is the code:

<?php
/*
Plugin Name: Disable Admin Bar
Version: 1.0
Author: Ben-Gad Rosental
Description: Disables The Admin Bar
*/

function remove_admin_bar() {
  show_admin_bar(false);
}
?>

Any idea?

Thanks.