1. Call someone and request a job (like Gerald)
2. Get in the car you want
3. Start the mission
4. drive the car to your garage and put it in
5. Exit the mission by going on your phone and clicking quit mission

2 Ways:

1 Way : With Tracker
1) get into the car you want
2) call ant contact that will give you a job (simeone)
3) accept the job
4) confirm the settings and play the mission
5) drive the car into the garage
if it says you cant put the vechile inside the garage go to this link: [ Register or Signin to view external links. ]

2 Way : Without Tracker
1) get into garage
2) grab the car you want to dupe
3) call simeone (you should still be in garage)
4) request a job
5) open the job (stay in vechile dont go to the job)
6) drive out the garage with the phone open
7) once you see the all black screen SPAM A
8) (you should be in a lobby)
9) wait 5 secounds in the lobby
10) Click B then wait 2 secounds
11) Click A
12) their should be 2 of the same cars and one of them is yours!

Hope I Helped ;)