If you want to Dupe any car follow this steps.

1 Have a friend who can join to your game.
2 Select the car that you want to duplicate, and take it off from your garage.
3 Call Gerald and ask him for a mission.
4 Accept the mission and inv your friend to it.
5 Goto 1st checkpoint and get the cocaine.
6 This is very important, so take care. Your friend must drive YOUR car and
walk away as far as he can from the you, meanwhile you are going to Gerald's house.
7 Give the cocaine to Gerald to complete the mission.
8 Once you finished the mission give a like/dislike to the mission and then chose "free mode" (I don't think it's the correct spelling, but anyways)
9 Your friend should have you car and you should have it too.
10 Your friend can save it in his own garage.

This glitch doesn't work with cars bought at "Legendary Motorsport"

Ah, I'm sorry for my crappy english, I'm living in Spain and English is not that common.

Anyways, enjoy dupping cars ;)