Doing this with doctors though would in a GP scenario, be pressure making with 10 minute slots that run over anyway. 5% over booking maybe acceptable, in a I don't mind waiting if everyone did miraculous turn up.
IDK the ins and outs of the specifics of my surgery's standby list, but it's not like an airline where there are 100 seats but they sell 110 seats and hope 10 people don't turn up. The GP isn't overbooked, the "correct" number of appointments are given out but, if people don't turn up on the day, or cancel last minute, they see if anyone can come in at short notice.
So there is a list of people who were given the "next available appointment" wen they booked in to see the GP, which might be for next Friday, but then today Mrs Jones calls at 11:00 to say that she can't make her 13:30 appointment. Rather than that appointment going to waste, the receptionist will get on the phone to Mr Smith, who is top of the standby list, and say "Hi, this is You have an appointment for next Friday, but we have had a cancellation today, would you be able to come in for 13:30? You can make it, great we'll see you then/You can't make it, no problem, we'll see you next Friday and get back to you if any more cancellations open up before then". If they can't make it then they move to the next person on the list and so on until the appointment is filled.
Or say Mrs Jones just doesn't turn up for their 13:30 appointment, but at 13:28 Mrs Cooper and her 7 yo DD who have a double appointment at 13:40 are already in the waiting room .... they will be bumped forward and the receptionist will call Mr Smith to say ""Hi, this is You have an appointment for next Friday, but we have had a late cancellation today, any chance you can come in within the next 20 minutes or so? Great, if you can get in by 13:50 we can give you that slot but if you can't make it, no problem, we'll see you next Friday anyway"
I would assume that if the GP has been really held up by over-running appointments or it was an unusually busy day and there is maybe one or two cancellations, they might use the freed up time to let the doctor catch up with the appointments that were already waiting before they start re-filling the cancelled slots but I like the fact they are trying not to waste time and give patients that option.