Duplication technique, RP suggestions

[TheC]

Hello everyone, me and my friend have scoured for hours on YouTube to only find half of them don't work and don't give the results we want, I've searched the forums but I want to hear suggestions from people on how to do what we want.

 

We want to duplicate our cars in order to make some cash, I know there is various methods such as the apartment switch and the cargo bob trick which both worked in one way where the car was duplicated and we managed to get one each but kept running into stuff like the car being locked and just wasting our time.

 

Also we would like to know the best method in order to rank up quickly, what mission, technique would you guys suggest.

 

Thanks