This code should be runned only once

fix_featured_listings_meta.php

<?php

/**
 * Delete all the duplicated `_featured` meta data
 **/
function clear_listings(){
	global $post;

	$q_args = array(
		'post_type' => 'job_listing',
		'posts_per_page' => '-1',
		'meta_query' => array(
			array(
				'key' => '_featured',
				'compare' => 'EXISTS'
			)
		)
	);
	$query = new WP_Query($q_args);
	while ( $query->have_posts() ) { $query->the_post();
		$f_meta = get_post_meta( $post->ID, '_featured', 1 );
		delete_post_meta( $post->ID, '_featured' );
		update_post_meta( $post->ID, '_featured', $f_meta );
	}
}
add_action('init', 'clear_listings');