_wp_copy_post_meta()

_wp_copy_post_meta( int $source_post_id, int $target_post_id, string $meta_key )

function _wp_copy_post_meta( $source_post_id, $target_post_id, $meta_key ) {

	foreach ( get_post_meta( $source_post_id, $meta_key ) as $meta_value ) {
		/**
		 * We use add_metadata() function vs add_post_meta() here
		 * to allow for a revision post target OR regular post.
		 */
		add_metadata( 'post', $target_post_id, $meta_key, wp_slash( $meta_value ) );
	}
}