1. Two garages (preferably Unit 2 Popular St. and Murietta Heights) - Unit 2 needs to have at least 1 empty slot. Murietta Heights needs to be full. (use Elegy's or random cars to fill the garage).
2. You need one garage to have the car you want to duplicate in it (I'm using Unit 2)
3. Proximity Mines (buy them at your local Ammunation)

1. Take the car you want to duplicate out of Unit 2 Popular St. and drive it over to Murietta Heights (DO NOT DRIVE IT INSIDE MURIETTA HEIGHTS)
2. Place a proximity mine on/inside the BLUE dot outside of Murietta Heights.
3. Get in the car you want to duplicate and simply hold down the "Dirve-Forward" button towards the BLUE dot where your proximity mine is (you might have to go to the left of it by just a tad) and you should blow up and die. That's great - it means it worked so far.
4. Take any car off the street and head over to Unit 2 (DON'T DRIVE IN)
5. Call up Mors Mutual and request the car you were duplicating.
6. Once you see a car icon apear on your map, drive into Unit 2 with the street car.
7. It should just normally drive you in (this is why you need a couple empty spots)
8. Go to the blue dot at the end of your garage and move the car you just drove in to another spot.
9. Go outside, open phone, job list, start any mission from Lester, Gerald, etc.
10. Drive that one over to LS customs and change something on it (such as the license plate) - then drive it into your garage!
9. Congratulations! You have now duplicated your car - enjoy!