It's likely unnecessary. Most rental companies invite to provide your inbound flight information in your reservation. Agents may monitor your flight's arrival time if you do not pick up the car at or near the reserved time, and hold the car for you until your flight arrives. But a reservation is never a guarantee of there being a car for you, because some other renters might not return their cars on time. Some renters now book a back-up reservation with a different rental company to guard against that eventuality. If you try to book two or more reservations on the same day with the same rental company -- say a hour apart -- the additional reservations might be cancelled. More importantly, if you make a habit of it, I could see a rental company putting you on their DNR (do not rent) list for "spoiling" their inventory with your multiple reservations.