[appfuse-user] add user with the same username to Set

[maker su]

Hi there,
I create a action with a field :
private Set<User> users = new HashSet<User>();

I add 2 users with differenet email to the Set<User>:

  user = new User();
  user.setUsername("Unknown");
  user.setEmail("a...@gmail.com");
  getUsers().add(user);

  user = new User();
  user.setUsername("Unknown");
  user.setEmail("d...@gmail.com");
  getUsers().add(user);

but it can only add 1 user which is the first one.
It seems only can add user to set when username is different.
How to do if I would like them with the same name "unknown"?

Please hint.
Sorry,it's may not much releated to appfuse user question.

Thanks,
Maker Su