code: public void assertOneOrLess(final String paramName,
              final ParameterList parameters) throws ValidationException {
  
          if (parameters.getParameters(paramName).size() > 1) {
              throw new ValidationException(ASSERT_ONE_OR_LESS_MESSAGE, new Object[] {paramName});
          }
      }
docs: Ensure a parameter occurs no more than once.
  
  @param paramName
  the parameter name
  @param parameters
  a list of parameters to query
  @throws ValidationException
  when the specified parameter occurs more than once