The private dining room is promised twice
An enquiry comes in by phone, another by email, and two people each pencil in the private room for Saturday afternoon. The clash is found when the second party is already on its way.
Make the private dining room a sub-calendar that does not allow overlaps. The second booking is refused when it is saved, and the person saving it sees what it collides with. The terrace and the bar can follow the same rule, or not, room by room.
The tasting is refused on save, and can be offered the terrace instead.