I only dup'd it from a friend once. I use the car glitch (sorry QS) and sell it over and over.
And for quick level up, racing is indeed the best way. If you go slow on the first lap in a 3 lap race at the prison track (when I say slow, make the first lap take at least 1:30) and increase speed on each lap, you will get max RP's.
Using slipstream/drafting and exchanging overtakes with friends or other racers before each checkpoint also gains extra RP.