It's because during the time users on Stripe website for processing payment, that registration is not confirmed yet, so the system still allows other users to perform registration
To reduce this over booking, go to Events Booking -> Configuration, set Count Awaiting Payment Registration Time (Minutes) config option to 2 for example
So the system will also count the pending registrations in last 2 minutes while checking to see if the event is open for registration
That won't solve the issue 100%, but it would help reduce the change this happens
Tuan
