Problem with template_redirect

Hello; I have written a plugin that loads a different template if a variable “tpl” is set in the url :

<?php
/*
Plugin Name: Template Changer
Description: Having a specific template if $_REQUEST[‘tpl’] is set.
Author: Gordie (based upon the script by Kaf Oseo)
Version: 3
*/
function template_changer() {
if ($_REQUEST[‘tpl’]) {
$tpl_file = TEMPLATEPATH . “/” . $_REQUEST[‘tpl’] . ‘.php’;
if ( file_exists($tpl_file) ) {
include($tpl_file);
}else{
return;
}
}else{
return;
}
}

if (!is_admin()) {
add_action(‘template_redirect’, ‘template_changer’);
}
?>

The template loads since it doesn’t display anything if the content of my tpl (popup.php actually) is
<?php exit;?>

But If I put a real template code;
It does not work :
For example; If I duplicate the content of the template “single” (just changing the main class; to check if it works); It shows the page twice : once with popup.php and the other with the regular single template !!!

How can I do ?