Avoid duplicate Contact Subtypes

Adding duplicate Contact Subtypes to a Contact isn't possible via the CiviCRM admin UI or the API Explorer and therefore shouldn't be possible when calling the API in code - but it is:

$result = civicrm_api3('Contact', 'get', [
  'id' => 205,
  'contact_sub_type' => ["Staff", "Staff", "Staff"],
]);

This results in:

[contact] => Array
  (
    [contact_id] => 205
    [contact_type] => Individual
    [contact_sub_type] => Array
      (
        [0] => Staff
        [1] => Staff
        [2] => Staff
      )
    ... more data ...

In CiviRules, when multiple rules which add the same Contact Subtype run, duplicate Contact Subtypes are added to the Contact. This shouldn't happen.

PR to follow.