code: public U duplicateStream(boolean lastStream) {
          if (!processor.isDuplicable()) {
              throw new IllegalStateException("duplicator is closed or last downstream is added.");
          }
          return doDuplicateStream(new ChildStreamMessage<>(this, processor, lastStream));
      }
docs: Creates a new {@link U} instance that publishes data from the {@code publisher} you create
  this factory with. If you specify the {@code lastStream} as {@code true}, it will prevent further
  creation of duplicate stream.