Double Bookings

Someone travelling to a class that isn't happening is the worst version of this, so it goes above the queue. Sounds like the availability check and the write aren't in one transaction. Looking now.

Confirmed, and it is exactly that. Two people on the last place both pass the check before either of them writes.

The seat is now claimed as part of the same write, so the second person is told it has gone rather than being confirmed into a class that is full. Testing it against a simulated rush before it goes out.