fb_og_desc filter is adding, not replacing

I’m trying to customize the og:description to pull from a custom meta description field. My code is adding the correct og:description, but it’s not replacing, so the og:desc is also appearing further below, pulling in the post title.

Sample page:
https://blogs.getty.edu/iris/getty-voices-our-l-a/

This is the code I’m using:

add_filter('fb_og_desc', 'custom_og_description');
function custom_og_description() {
	/* Get the current post ID. */
	$post_id = get_the_ID();

	/* If we have a post ID, proceed. */
	if ( !empty( $post_id ) ) {

		/* Get the custom post description. */
		$desc = get_post_meta( $post_id, 'iris_post_metadescription', true );

		/* If a post description was input, change the og description. */
		if ( !empty( $desc ) )
			echo "<meta property=\"og:description\" content=\"" . $desc . "\" />\n";

	}
}