You have to be in the car you want to duplicate and the other person has to hope in the passengers seat the person in the passengers seat has to ring gearld and request for a job when the passenger gets the job he has to accepted the job and play the mission he will spawn in your car and they have to drive it in there garage after that they have to walk out and kill them self 2 to fail the mission select free mode then go into your garage there is the duplicated car!
(you can only do it with some cars)

1. Get in passenger seat of vehicle.
2. Call Gerald, Simeon, or Lester and request a job.
3. Start mission up by yourself.
4. Drive vehicle into your garage.
5. Kill yourself two times during the mission.
6. Enter free roam.