Programmatically duplicating a WordPress post

aux-functions.php

<?php

  /**
   * Duplicates a post & its meta and it returns the new duplicated Post ID
   * @param  [int] $post_id The Post you want to clone
   * @return [int] The duplicated Post ID
   */
  function duplicate($post_id) {
    $title   = get_the_title($post_id);
    $oldpost = get_post($post_id);
    $post    = array(
      'post_title' => $title,
      'post_status' => 'publish',
      'post_type' => $oldpost->post_type,
      'post_author' => 1
    );
    $new_post_id = wp_insert_post($post);
    // Copy post metadata
    $data = get_post_custom($post_id);
    foreach ( $data as $key => $values) {
      foreach ($values as $value) {
        add_post_meta( $new_post_id, $key, $value );
      }
    }

    return $new_post_id;
  }