On one an account simultaneously there can only be one operation which takes time on the calendar.
It includes checking that this time is available.
All other such operations await their turn.
This is achieved through system locks.
This eliminates the possibility of an accidental overbooking.
The administrator can purposefully do a rebooking.
To do this, the Overbook checkbox is included in the booking parameters.
With the checkbox set, you can take more seats than is actually available.