Double-booking

Double-booking is two appointments occupying the same staff member at the same time. It happens when availability is checked before a booking is written but nothing stops a second booking landing in the gap between the check and the write.

Also called: overlapping appointments, two clients same slot

The failure is a race, not an oversight. Two clients open the same slot, both see it free, and both confirm within a second of each other. Any check performed before the write can be overtaken by the other booking.

Filtering the times shown in the interface is not protection. It reduces how often the race is entered without changing what happens when it is.

The only reliable fix is a rule inside the database that refuses to store the second overlapping booking, so the loser of the race gets an error instead of an appointment.