Have a full garage and have the money required to buy it (you will not actually spend this money)
2) Go online and buy the car you want
3) When the menu comes up for which car do you want to replace DO NOT click on it but hover over the car you want to get rid of
4) once hovering over the car you want to get rid of for your new car go press start, go to online and hit leave gta online
5) when you spawn in single player there should be the message that pops up every time you want to replace a car, press ok, and then ok again
6) go into gta online and the car should be sitting in your garage in place of the one you got rid of and you will still have your money.
If you dont have enough money do the LSC glitch to get some and then you can always dump the money if you dont want to have it
Guys i know this isnt exactly about duping cars but it can be helpful for the guys who request premium cars on here all the time and want nice cars but don't want to spend all of their money on one.

credit: http://www.youtube.com/watch?v=HKj89Xd67qc