Customize any car as desired, then switch to a different character, drive by the character in the customized car, and hang out with him. Then, get out of your vehicle, and drive their customized car. When you return with the customized car after hanging out with the other character, there will be another of the same customized car. Get out of the car, and take the duplicated customized car. Then, switch back to the other character, and take the original customized car.