code: private boolean checkDirective(String directive) {
          if (directives.containsKey(directive)) {
              throw new IllegalArgumentException("Duplicate cache-control directive in configuration file");
          } else if (Directive.get(directive) == null) {
              throw new IllegalArgumentException("Missing or incorrect cache-control syntax in the configuration file");
          }
          return true;
      }
docs: Checks for duplicate directive values and correct syntax
  @param directive
  @return