Hopefully this will help.
Step 1: Request a Job
Step 2: Go to the menu where your about to accept the job
Step 3: Enter the desired car to duplicate
Step 4: Confirm the job and as soon as you confirm the job step on the gas pedal (Have a friend watch your car exit the garage)
Step 5: Go to the menu where you invite people to the job (1:06 on the video)
Step 6: Back out of the job
Step 7: Get back in your car and drive to your garage (You will be teleported far away from your garage)
Step 8: If you see your car, the glitch has worked successfully. If not, repeat steps 1-7 carefully.
Step 9: Rinse and Repeat.