Two phone calls at the same moment, a guest booking online while a waiter adds a walk-in: that's where double bookings come from.
In TAVIVA every occupied table is recorded with its time range, including table reset time. A PostgreSQL constraint prevents two ranges of the same table from overlapping: it's not an interface rule, it's a system guarantee.
On top of that guarantee, the availability engine picks the best table, respects arrival limits per slot and handles services past midnight and daylight saving changes.
In our automated tests 25 simultaneous requests compete for the same 4 tables: exactly 4 are accepted. Every time.