These CAN be duped, you just paint the car then send it to the dupe account, rather than sending the paint, then in the other account you delete and restore your gamesave, and you'll have the paint back. Then you just send the car from the other account back to the main account, and you have the car and the chip.
REMEMBER, you can't do this with cars that are DLC, stealth, chromeline, or over 1,000,000 credits.