If you have a single page already, you shouldn't be using a wp_query as well.
Instead of these lines:
<?php $args = array('post_type' => 'listing');
$loop = new WP_Query( $args );
while ( $loop->have_posts() ) : $loop->the_post(); ?>
Use these lines:
<?php
// Start the loop.
while ( have_posts() ) : the_post(); ?>
