Replace Yoast og:image in single custom post type

i’m trying to replace the custom yoast og:image in a specific post type to use a custom meta field instead of the featured image. What i got so far is:

function my_wpseo_opengraph() {
global $post;
if (is_singular('job_listing')) {
    $og_image = get_post_meta($post->ID, 'pp_job_header_bg', TRUE); 
    if ( ! empty ( $og_image ) ) {

        $GLOBALS['wpseo_og']->image($og_image);
    }

}
}
add_action('wpseo_opengraph', 'my_wpseo_opengraph', 29);

However, while viewing the source code, it seems that the code won’t replace the default og:image, it adds a second og:image.

How do i replace it?