This avoids any negative side effect from a race condition where two
clients request a game to start within a short time window of each
other. When one prompt is active, the server will ignore any further
incoming requests to start a new prompt, (until the active one is
completed).
if (! prompt)
return false;
+ /* Ignore any start request that comes in while a prompt is
+ * already being played. */
+ if (this.state.active_prompt)
+ return false;
+
this.state.active_prompt = prompt;
this.broadcast_event_object('start', prompt);