Example: There is a legit mark x with orig documents auctioned by customs and that car worth 1 cr
Doubling means an ncp car with same color as above is copy pasted with the chassis and other credentials of the above legit car and now there are 2 cars of same number. Excise record will show legit data but in laboratory test or if both cars are in one place they will be caught.
If you are a law abiding citizen stay away from such car.