code: @Override
      public void validate(IValidatable<String> validatable) {
          boolean hasError = false;
          String s = validatable.getValue();
          String[] elements = s.split(",");
          if (elements.length > 1) {
              List<String> nonDuplicatesList = new ArrayList<String>(new LinkedHashSet<String>(Arrays.asList(elements)));
              if (elements.length != nonDuplicatesList.size()) {
                  // have duplicates
              	hasError = true;
              }
          } else {
              elements = s.split("-");
              if (elements.length == 2) {
                  if (SelectIntervalPanel.getComparator(entityType, false).compare(elements[0], elements[1]) >= 0) {
                  	hasError = true;
                  }
              }
          }
          
          if (hasError) {
  			ValidationError error = new ValidationError();
  			String messageKey = Classes.simpleName(IntervalFieldStringValidator.class);
  			error.addKey(messageKey);
  			validatable.error(error);
          }
      }
docs: TODO wicket-6