Basically you back up your primary GT5 account's saved game data file to a USB stick, gift a car to a secondary PSN account you create, then reload the USB stick's GT5 data file (essentially putting the gifted car back in to your garage), then gift the car from the secondary PSN/GT5 account back to your primary. Et voila, 2 of the same car.
Or you can send the car to someone else (instead of your secondary account), then that way, you will still have the car even though you'd gifted it.
Basically you back up your primary GT5 account's saved game data file to a USB stick, gift a car to a secondary PSN account you create, then reload the USB stick's GT5 data file (essentially putting the gifted car back in to your garage), then gift the car from the secondary PSN/GT5 account back to your primary. Et voila, 2 of the same car.
Or you can send the car to someone else (instead of your secondary account), then that way, you will still have the car even though you'd gifted it.