Personally, without some sort of "safe" trading system, where you both send your cars to a neutral account, and once each has been received, the cars are sent to the corresponding account.
"duping" as you call it makes perfect sense.
IMO there should have never been trading in the first place, but since it's here with no way of keeping it fair, the latter must follow.
and yes I do trade, Because it's my choice.
It's comical how people will let B-spec run for a whole day and then say that duping takes away form their racing time.
Maybe a case of the pot calling the kettle black......................