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This glitch will work best if you have at least $250k to start with. However, you only need around $2k to start the glitch off, and you can keep getting richer from there.
I also recommend having two 10 car garages.

1. If you have two garages, fill up one garage with 10 cars (the max) from the street and fill up your other garage with 9 cars (1 less than the max) from the street. If you only have one garage, fill it up with 9 cars (1 less than the max).
NOTE: Having a garage full of purchased cars does not work for this glitch. They need to be from the street and have no insurance on them.

2. Find a car off the street that will sell good (Patriot, Buffalo, Baller, etc.).
3. Drive it into LSC and upgrade it as much as you can. Once you are done upgrading your car, drive out of LSC.
4. Drive back up to LSC and put your two front wheels at the door.
5. Call your Mechanic and once the menu is up drive fully into LSC.
6. Select a car from the garage that this car will go in (which is the garage that has 1 less car than its max).

Do steps 7 and 8 quickly and do them immediately after Step 6!
7. Press Right on the D-Pad, repair the vehicle if needed and scroll down to "Lost/Theft Protection".
8. Put a tracker on the car and spam B/Circle to back out of LSC.
9. Start spamming Y/Triangle during the exit cutscene and your character should get out and then get right back in the car.
10. Drive the car into your garage.
11. You have successfully duplicated your car!

To do this again, simply destroy one of the vehicles in your garage and start from Step 2.