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list-templates.php

<?php
/*
 * Template Name: [ADMIN] List templates
 */

?>
<h2>Usage</h2>
<p>Append <code>?template=template-name.php</code> to this page URL</p>

<h2>Pages using <code><?php echo $_REQUEST['template'] ?></code></h2>
<?php

$pages = get_pages(array(
    'meta_key' => '_wp_page_template',
    'meta_value' => $_REQUEST['template'],
));

if(!empty($pages))
{
    foreach($pages as $page): ?>
        <p><a href="<?php echo get_permalink($page) ?>"><?php echo $page->post_title ?></a></p>
<?php
    endforeach;
}