Let's assume you accept one order / customer per timeslot.
The way FoodNow.ie work is that when a customer selects a timeslot, that timeslot is reserved for 5 minutes. During that time, no other customer can choose that timeslot. When the customer completes their order, the reservation is removed and the timeslot is permanently blocked.
If the customer does not complete their order in 5 minus, the reservation is removed and the timeslot is available to another customer.
If another customer selects the timeslot and the original customer then completes their order, both customers will have reserved for the same timeslot.
While uncommon, this scenario does happen from time to time.