Drupal 7 create user class doesn't support multiple creates

If you try to call CRM_Utils_System_Drupal::createUser more than once in one process the drupal caching causes it to bypass validation & for the password to be incorrect as a result.

Multiple creates aren't available in core yet but it makes sense for the underlying function to support it as it is certainly desirable for creating CMS user to be added as an action in future (for now I am doing via rules / civicrm_entity module)