The door’s reader can be programmed to open it when it interrogates any one of several serial numbers, so you could have multiple cards that open the door. You would not necessarily need to duplicate serial numbers as a back-up.
Keep in mind that most short-range RFID systems are based on open standards, and that it would be easy to create a device that would transmit millions of random numbers to the reader in the car door, thereby allowing a thief to open that vehicle. If you are serious about this deployment, you will need to look into encryption in order to ensure that the system is safe.
—Mark Roberti, Founder and Editor, RFID Journal