This is the simplest guide to duping cars:
For unlatched retail version see below on windows XP having installed the game in the default directory right click on the start button and press explore. Find the C: drive then open it. Then find a folder called program files open that, then open a folder called Activision Value. After that open a folder named 'save' then find you respective save game. 000 means slot 1 in the saved game slot 001 means slot2 and so on. then copy a file called playerCar0 (the number is the slot which the car is in, in your car lot, again playerCar0 is your first car). If you want to sell your cars then do not copy the playerCar*.1, playerCar*.2 and so on. If you want to keep the cars you dupe then copy these files and rename them as appropriate, these are the texture files.

For the patched version (2.1.9r5 and 2.2.1)

Do the same as above but instead open the save file then press carrier and open it then open the respective career folder and copy the cars.