How can I duplicate categories and tags?

add_action( 'save_post', 'add_tag_based_on_category' );

function add_tag_based_on_category( $post_id ) {
    $args = array( 'fields' => 'names' );
    $categories = wp_get_post_categories( $post_id, $args );
    $cat_names = implode( ', ', $categories );
    wp_set_post_tags( $post_id, $cat_names, true );
}