Adding a user with the same ID to multiple rooms

Adding a user (or users with the same ID) to multiple rooms should fail.
Use case:
game.createRoom('test');
game.addPlayer('test', 'a');

game.createRoom('test2');
game.addPlayer('test2', 'a');

The issue identifies game.createRoom and game.addPlayer as the relevant entry points; trace how player IDs are stored across rooms and how addPlayer reports failures. Add coverage for the shown two-room case and confirm that adding the same ID to the second room fails.

Written by the indexing model from the issue text.