Potential Double booking?

Trialing your appointment system and it seems to be what we need for parents evening bookings. However during tested I noticed that it was possible to double book a slot. If more than 1 user selects the drop down menu at the same time (we have a single drop down) and selects the date and time it does not block the appointment being booked. I have it set to confirm upon selection but it still can allow double bookings. I am not sure if in reality it would happen but I do worry about this aspect.

Is there a way that once the user actually submits the appointment, other attempts can be blocked so that no one can book the same (without refreshing the page)?
The test I carried out was selecting the same items at the same time on a phone and laptop, then confirmed one and left the other for 30 seconds and then confirmed it which allowed me to make a double booking.

I hope that makes sense.

Any help would be much appreciated.

Good Job

Steve