How to Add a “Duplicate Post” Feature to WordPress (No Plugins Needed)

/**
 * Duplicate post as draft (STABLE VERSION)
 */
add_action( 'admin_action_rd_duplicate_post_as_draft', function() {

    if ( ! isset( $_GET['post'], $_GET['duplicate_nonce'] ) ) {
        wp_die( 'Invalid request.' );
    }

    $post_id = absint( $_GET['post'] );

    // Verify nonce
    if ( ! wp_verify_nonce( $_GET['duplicate_nonce'], 'rd_duplicate_post_' . $post_id ) ) {
        wp_die( 'Security check failed.' );
    }

    $post = get_post( $post_id );

    if ( ! $post || ! current_user_can( 'edit_post', $post_id ) ) {
        wp_die( 'Post not found or permission denied.' );
    }

    // Create duplicate post
    $new_post_id = wp_insert_post( [
        'comment_status' => $post->comment_status,
        'ping_status'    => $post->ping_status,
        'post_author'    => get_current_user_id(),
        'post_content'   => $post->post_content,
        'post_excerpt'   => $post->post_excerpt,
        'post_name'      => '', // Let WP generate unique slug
        'post_parent'    => $post->post_parent,
        'post_password'  => $post->post_password,
        'post_status'    => 'draft',
        'post_title'     => sprintf( '%s (Copy)', $post->post_title ),
        'post_type'      => $post->post_type,
        'menu_order'     => $post->menu_order
    ] );

    if ( is_wp_error( $new_post_id ) || ! $new_post_id ) {
        wp_die( 'Failed to create duplicate post.' );
    }

    // Copy taxonomies
    $taxonomies = get_object_taxonomies( $post->post_type );
    foreach ( $taxonomies as $taxonomy ) {
        $terms = wp_get_object_terms( $post_id, $taxonomy, [ 'fields' => 'ids' ] );
        if ( ! is_wp_error( $terms ) && ! empty( $terms ) ) {
            wp_set_object_terms( $new_post_id, $terms, $taxonomy );
        }
    }

    // Copy meta (safe approach)
    $meta = get_post_meta( $post_id );

    $skip_meta = [
        '_wp_old_slug',
        '_edit_lock',
        '_edit_last',
        '_wp_trash_meta_status',
    ];

    foreach ( $meta as $key => $values ) {
        if ( in_array( $key, $skip_meta, true ) ) {
            continue;
        }

        foreach ( $values as $value ) {
            add_post_meta( $new_post_id, $key, maybe_unserialize( $value ) );
        }
    }

    // Copy featured image
    $thumbnail_id = get_post_thumbnail_id( $post_id );
    if ( $thumbnail_id ) {
        set_post_thumbnail( $new_post_id, $thumbnail_id );
    }

    // Redirect to edit screen
    wp_redirect( admin_url( 'post.php?action=edit&post=' . $new_post_id ) );
    exit;
} );


/**
 * Add Duplicate link
 */
add_filter( 'post_row_actions', 'rd_duplicate_post_link', 10, 2 );
add_filter( 'page_row_actions', 'rd_duplicate_post_link', 10, 2 );

function rd_duplicate_post_link( $actions, $post ) {

    if ( current_user_can( 'edit_post', $post->ID ) ) {

        $url = wp_nonce_url(
            admin_url( 'admin.php?action=rd_duplicate_post_as_draft&post=' . $post->ID ),
            'rd_duplicate_post_' . $post->ID,
            'duplicate_nonce'
        );

        $actions['duplicate'] = '<a href="' . esc_url( $url ) . '">Duplicate</a>';
    }

    return $actions;
}