I'm sorry, but if other people duping ruins your game that's your problem. Makes no sense. This is a game not "work" or real life. The method is out there, if you choose not to use it and gather cars "legitimately" then that is your call. Doesn't affect your game, doesn't affect the imaginary "economy" some people have created in their minds. Please stop using real life economics for a game that has no market place, no auction house and no player to player currency. You can still collect cars just like you always have. Doesn't affect your GT Life mode in the least. I could just as easily say those of you who gifted cars back and forth as trades were abusing the game.
As far as online, I've not been made privy to any accounts of duped cars being faster than ones earned "the right way". Someone who is gifted a Zonda and hasn't put in the hours is still going to be crap at driving. Or does duping increase user skill? If it really bothers some so much, pack your list with "real" players from the forum and trade the old fashion way.
Really, what do you care how others play their game?
And please, before you start preaching at me, I don't dupe. Don't even know how. I'm too lazy to do the birthday thing, so I'm surely not researching methods to do this. I just can't fathom how this "ruins" the games of "real" players. Maybe it does, but only in your head. Please let it go.