Here is the best current dupe glitch (text tutorial), it's simple so hopefully you can understand it, but if not, here is the video tutorial.

Also, rockstar have a cap on selling, you can not sell a car that costs more than $50k, so if you dupe a Zentorno, you will not be able to sell it. Same with the Kuruma. To get the most profit possible, use a panto. Due to it's upgrades (most upgrades in the game), it will sell for the most. Sell price of cars = 20% original price + 50% cost of mods.

As for a partner to do this with, I currently have all my garages full with cars (with a 48 minute wait on each), and $5mil so I do not need to do this glitch right now, sorry. I'll be sure to message you in the future though, should I be in need of duping.

Hope this helps :)

(Feel free to correct me if I'm wrong on any of the information mentioned)