Duping most certainly DOES have an impact in online...
With the exploit, you can now have multiple upgrade versions of all your favorite cars ready to go for whatever restrictions the lobby creator picks. Those that stay honest to the game probably can't afford to do that.
Plus, with duping, damaged or aging cars don't have to have repairs PAID for, simply replaced with the dupe.
There's a LOT more to that exploit than meets the eye.
Or simply equip/unequip parts?
The one real fix I'd like to see is a patch that disallows a 'gifted' car from entering online. I know, I know, this would queer the pitch for the few that WOULD like to use the feature legitimately, but at this point, it looks like we are going to have to take the bigger picture into account.
Having innumerable players online in cars they haven't EARNED or BOUGHT (you need to be at a particular level to buy certain cars) lowers the likely driving standards, and dilutes the experience of racing competent opponents.
If not this, then perhaps some lobby filter for Level, or passing all S-Class License Tests, or the like. But wait! you are all going to say, why not simply race your Friends? For all those decrying that the game should not have ANY limitations, this must be obvious. Why should I have to wait for THEM to be available JUST to get a fair and decent race? The whole POINT is, log on at any time, night or day, and race online against whoever happens to be around.
But the game seems to have given up on moderating itself to ensure fairness, and left it to US to have to do it for ourselves, when quite reasonable restrictions could be placed in the hands of Lobby creators to filter entrants by experience, Level and License progress.
The hacked game save has some pretty deep implications for the longterm health of the game. Once you have progressed as far against the AI as you are willing to do, what is left BUT online..?
I imagine Sony are taking this VERY seriously, I hope PD is, too...
What would be even better is to restrict cars to players by their A-Spec level, offline and online. If you're an A-Spec Zero (0) and Bob is a B-Spec Hero (40), Bob has the right to race an X2010 and certainly not you. This will also help with the gifting issue of, say, duplicated high level cars and kiddies getting super powered cars while being an A-Spec Zero. Sure OFFLINE you can test cars higher-level-than-your-own, but can't race them to progress yourself.
Speaking of duplicated cars...
This brings me back to Pokemon, and I'll tell you why. In Pokemon, when you captured a Pokemon, it is OT/*Your Name* (Original Trainer/*Your Name*) and given a 12 digit ID (6 visible, 6 hidden). That way, two players under the same name can't share Pokemon that they can call their own without people knowing it's been traded.
NOW, back to why Pokemon is so important. I think every car earned, by ticket or purchased, should have their name inscribed as the first original owner of the car, along with a, say, 12 digit hidden ID to the car. With that, you have a completely unique car.
Now how will this solve duping? Easy. Once it is detected that two cars of the same registered name and ID are present, that car is locked/removed. Same can go for Paint Chips/Horns/Car Parts.
Why have the hidden ID if all PSN names are unique themselves? Well, if you bought two Ferrari Enzo's, they both could have two different ID's without both being locked/removed, because (Guess What?) you earned them without duplicating.
Easy when someone who can think things through reveals himself.
Immature people + hacks + online = no fun for serious players. If I were in a goofy mood it could be fun to drive a Vitz that handled like an X2010 though haha! 
