Please +rep and thank me if it worked.
Tutorial:
"In your garage get in a car and wait until he/she starts the engine,
Then
Press Y or Triangle
Then
Press right trigger or R2
When the screen goes black keep holding Triangle"

This must be done quite fast but not to fast and not sloppy. You need to press Y the second time and DON'T need to have a full garage.

You must see the animation of the character start to get out of the car (door cracks open) then the screen goes black and the garage door sounds. Then you'll spawn in between the garage and the garage door outside of your car. Get in the car and drive a block or two away. Call the mechanic to bring you the same car. Now when you see your original car pull up sell the duped one your in. Then go back to your original and bring it back to your garage.

You can also drive the duped car back into your garage and keep it or sell it after.

Thank you for reading.