As the comment of @Elliott-Frisch, You need to add username
to clientList after you check its validity. You may also simplify clientList.size() > 1 && clientList.contains(username))
as clientList.contains(username)
You can just put clientList.add(username);
before the line you create the client object:
clientList.add(username);
//try creating a new Client with GUI
client = new Client(username, this);
//test if we can start the Client