Cannot deal with multiple iterations of same form on one page
You could also add something like this to your form class

 /**
  * Keep track of how many times the form
  * is placed on a page.
  *
  * @var int
  */
protected static $instanceId;

/**
 * {@inheritdoc}
 */
public function getFormId() {
  if (empty(self::$instanceId)) {
    self::$instanceId = 1;
  }
  else {
    self::$instanceId++;
  }

  return 'my_form_id_' . self::$instanceId;
}