Don't allow players in same room to have the same pseudo

[delphinemillet]

[enhancement]
It could be nice to prevent a user to join a room if a user in this room has the same pseudo

[fabienjuif]

I aim at having an Elo system (https://fr.wikipedia.org/wiki/Classement_Elo)
It means players would connect themself to the game (with credentials).

At this moment, I would test pseudo unicity.