code: private static boolean areAttachmentLabelsUnique(List<String> labels) {
      Map<String, String> map = new TreeMap<String, String>(String.CASE_INSENSITIVE_ORDER);
      for (int i = 0; i < labels.size(); i++) {
        map.put(labels.get(i), labels.get(i));
      }
      if (labels.size() == map.size()) {
        return true;
      } else {
        return false;
      }
    }
docs: This method checks if strings in a list are unique
  
  @param labels list of attachments labels
  @return true or false