How to Duplicate Posts and Add a Duplicate Button in WordPress

<?php

/*
 * Function for post duplication. Dups appear as drafts. User is redirected to the edit screen
 */
function rd_duplicate_post_as_draft(){
  global $wpdb;
  if (! ( isset( $_GET['post']) || isset( $_POST['post'])  || ( isset($_REQUEST['action']) && 'rd_duplicate_post_as_draft' == $_REQUEST['action'] ) ) ) {
    wp_die('No post to duplicate has been supplied!');
  }
 
  /*
   * Nonce verification
   */
  if ( !isset( $_GET['duplicate_nonce'] ) || !wp_verify_nonce( $_GET['duplicate_nonce'], basename( __FILE__ ) ) )
    return;
 
  /*
   * get the original post id
   */
  $post_id = (isset($_GET['post']) ? absint( $_GET['post'] ) : absint( $_POST['post'] ) );
  /*
   * and all the original post data then
   */
  $post = get_post( $post_id );
 
  /*
   * if you don't want current user to be the new post author,
   * then change next couple of lines to this: $new_post_author = $post->post_author;
   */
  $current_user = wp_get_current_user();
  $new_post_author = $current_user->ID;
 
  /*
   * if post data exists, create the post duplicate
   */
  if (isset( $post ) && $post != null) {
 
    /*
     * 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
     */
    $taxonomies = get_object_taxonomies($post->post_type); // returns array of taxonomy names for post type, ex array("category", "post_tag");
    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 just in two SQL queries
     */
    $post_meta_infos = $wpdb->get_results("SELECT meta_key, meta_value FROM $wpdb->postmeta WHERE post_id=$post_id");
    if (count($post_meta_infos)!=0) {
      $sql_query = "INSERT INTO $wpdb->postmeta (post_id, meta_key, meta_value) ";
      foreach ($post_meta_infos as $meta_info) {
        $meta_key = $meta_info->meta_key;
        if( $meta_key == '_wp_old_slug' ) continue;
        $meta_value = addslashes($meta_info->meta_value);
        $sql_query_sel[]= "SELECT $new_post_id, '$meta_key', '$meta_value'";
      }
      $sql_query.= implode(" UNION ALL ", $sql_query_sel);
      $wpdb->query($sql_query);
    }
 
 
    /*
     * finally, redirect to the edit post screen for the new draft
     */
     
    // wp_safe_redirect( wp_get_referer() ); // Option to not get redirected to new post edit page, but stay where one was. 
    // wp_redirect( admin_url( 'post.php?action=edit&post=' . $new_post_id ) ); // replace this line with the following
    wp_safe_redirect( admin_url( 'post.php?action=edit&post=' . $new_post_id ) ); // Security related edit: @link https://developer.wordpress.org/reference/functions/wp_safe_redirect/
    exit;
  } else {
    wp_die('Post creation failed, could not find original post: ' . $post_id);
  }
}
add_action( 'admin_action_rd_duplicate_post_as_draft', 'rd_duplicate_post_as_draft' );
 
/*
 * Add the duplicate link to action list for post_row_actions
 */
function rd_duplicate_post_link( $actions, $post ) {
  if (current_user_can('edit_posts')) {
    $actions['duplicate'] = '<a href="' . wp_nonce_url('admin.php?action=rd_duplicate_post_as_draft&post=' . $post->ID, basename(__FILE__), 'duplicate_nonce' ) . '" title="Duplicate this item" rel="permalink">Duplicate</a>';
  }
  return $actions;
}
 
add_filter( 'post_row_actions', 'rd_duplicate_post_link', 10, 2 );
add_filter( 'page_row_actions', 'rd_duplicate_post_link', 10, 2 ); // add the link to pages too