I have duplicated colors by trading one car back and forth between my multiple previously-existing PSN accounts (I have one for each major region so I can enjoy the best of all PSN stores), but never cars. Nor have I birthday glitched (when my real birthday comes up, I'll accept it) or done any sort of dupe-trading.
I only have one car in my garage that I did not earn or trade for, which was gifted to me by a friend.
I have been running the seasonal events on my non-main accounts, though, and then gifting cars bought with those credits to my main account. Also, re-running licenses and events to re-win prize cars that I foolishly sold from my main account. Not being able to earn the prizes multiple times is BS.