How to Duplicate Pages and Custom Posts Without a Plugin

// Add the duplicate link to action list for post_row_actions
// for "post" and custom post types
add_filter('post_row_actions', 'rd_duplicate_post_link', 10, 2);
// for "page" post type
add_filter('page_row_actions', 'rd_duplicate_post_link', 10, 2);

function rd_duplicate_post_link($actions, $post) {
    if (!current_user_can('edit_posts')) {
        return $actions;
    }
    $url = wp_nonce_url(
        add_query_arg(
            array(
                'action' => 'rd_duplicate_post_as_draft',
                'post' => $post->ID,
            ),
            'admin.php'
        ),
        basename(__FILE__),
        'duplicate_nonce'
    );
    $actions['duplicate'] = '<a href="' . $url . '" title="Duplicate this item" rel="permalink">Duplicate</a>';
    return $actions;
}


// Function creates post duplicate as a draft and redirects then to the edit post screen
add_action('admin_action_rd_duplicate_post_as_draft', 'rd_duplicate_post_as_draft');


function rd_duplicate_post_as_draft() {
    // check if post ID has been provided and action
    if (empty($_GET['post'])) {
        wp_die('No post to duplicate has been provided!');
    }
    // Nonce verification
    if (!isset($_GET['duplicate_nonce']) || !wp_verify_nonce($_GET['duplicate_nonce'], basename(__FILE__))) {
        return;
    }
    // Get the original post id
    $post_id = absint($_GET['post']);
    // And all the original post data then
    $post = get_post($post_id);
    $current_user = wp_get_current_user();
    $new_post_author = $current_user->ID;
    // if post data exists, create the post duplicate
    if ($post) {
        // new post data array
        $args = array(
            'comment_status' => $post->comment_status,
            'ping_status' => $post->ping_status,
            'post_author' => $new_post_author,
            'post_content' => $post->post_content,
            'post_excerpt' => $post->post_excerpt,
            'post_name' => $post->post_name,
            'post_parent' => $post->post_parent,
            'post_password' => $post->post_password,
            'post_status' => 'draft',
            'post_title' => $post->post_title,
            'post_type' => $post->post_type,
            'to_ping' => $post->to_ping,
            'menu_order' => $post->menu_order
        );
        // insert the post by wp_insert_post() function
        $new_post_id = wp_insert_post($args);
        // get all current post terms ad set them to the new post draft
        $taxonomies = get_object_taxonomies(get_post_type($post));
        if ($taxonomies) {
            foreach ($taxonomies as $taxonomy) {
                $post_terms = wp_get_object_terms($post_id, $taxonomy, array('fields' => 'slugs'));
                wp_set_object_terms($new_post_id, $post_terms, $taxonomy, false);
            }
        }
        // duplicate all post meta
        $post_meta = get_post_meta($post_id);
        if ($post_meta) {
            foreach ($post_meta as $meta_key => $meta_values) {
                if ('_wp_old_slug' == $meta_key) {
                    continue;
                }
                foreach ($meta_values as $meta_value) {
                    add_post_meta($new_post_id, $meta_key, $meta_value);
                }
            }
        }
        // finally, redirect to the edit post screen for the new draft
        wp_safe_redirect(
            add_query_arg(
                array(
                    'post_type' => ('post' !== get_post_type($post) ? get_post_type($post) : false),
                    'saved' => 'post_duplication_created'
                ),
                admin_url('edit.php')
            )
        );
        exit;
    } else {
        wp_die('Post creation failed, could not find original post.');
    }
}


// In case we decided to add admin notices
add_action('admin_notices', 'rudr_duplication_admin_notice');


function rudr_duplication_admin_notice() {
    // Get the current screen
    $screen = get_current_screen();
    if ('edit' !== $screen->base) {
        return;
    }
    //Checks if settings updated
    if (isset($_GET['saved']) && 'post_duplication_created' == $_GET['saved']) {
        echo '<div class="notice notice-success is-dismissible"><p>Post copy created.</p></div>';
    }
}