Overbooking when using PayPal

Hi.

We have an issue with overbooking when using PayPal payment because when you initiate payment over PayPal you are taken to external site and if more people do that at the same time, they are all allowed to complete booking even if all slots are filled.
This happen if you manage to click on Pay button before another user completes payment who is already on PayPal payment site before you.

Can you please check this issue? It is big problem with overbooking.