code: private List<T> makeUnmodifiableUniqueList() {
          Set<T> tmp = newHashSet();
          List<T> uniques = newArrayList();
          for (T t : getIterable()) {
              if (tmp.add(t)) {
                  uniques.add(t);
              }
          }
  
          sort(uniques);
          return unmodifiableList(uniques);
      }
docs: remove duplication if any