That doesn't work.
Duping...
1. Create a 2nd PSN account
2. Load GT5 on that account (you will need to set up your profile and buy a car, nothing else)
3. Send a PSN friends request to your main account
4. Go to your main account and copy your game save to a USB
5. While there, accept the friends request from your 2nd account
6. Boot the game
7. Send the car to your 2nd account via the gifting process
8. Quit the game, switch to your 2nd account and load the game
9. Your 2nd account will now have the car gifted from your main account
10. Send the car back to your main account via the gifting process
11. Quit the game and switch to your main account (do not boot the game)
12. Copy the game save on the USB back on to your PS3 (it will tell you there's a file with the same name and ask you if you want to over write it... click 'yes')
13. Boot the game
14. you will now have your original game save with the car and also have a gift of the same car from your 2nd account