There's a better method that doesn't require an impounded vehicle.
1) Order an Elegy.
2) Invite your friend to your property.
3) Have him sit in the passengers' seat of the car you wish to duplicate.
4) Tell your friend to switch to a singe player character, then remain at the screen that asks whether you are sure you want to leave online or not.
5) Drive out of your garage and tell your friend to back out of that screen immediately once the exiting garage animation begins. (At this point you will be able to tell if the glitch is working if your friend cannot move his camera, and if his map is entirely black with just the icons)
6) (Optional) Bang up your car for 20 seconds or so then drive back in.
7) Head to the car rearranging tool and swap your vehicle with the Elegy. Your car should duplicate.
8) Drive out of your garage with the car you originally were driving in. (Your friend should have been stuck in the garage entering animation, so tell him to leave the session.)
9) Sell the car, rinse and repeat.
I recommend doing this with an Adder (obviously, because it sells for the most money), and doing it in an invite-only session. I can confirm this works for both PS3 and Xbox because the YouTuber i saw this glitch from is on Xbox, and I tried it on PS3.