I hope many people don't double book! I rent about 35-40 cars a year.
Only once in over 5 years have the rental company not have a car for me
- and my plane arrived very late that time.
If a lot of people start double booking, I can see two disadvantages for
the traveler. Either I might end up getting a car from Rent-a-Wreck because
the car companies I prefer think all their cars are spoken for, or they
will start overbooking, like the airlines do to compensate for bookings
cancelled at the last minute.