In the Garage, the top right hand corner has a "Sort" option, which allows you to sort the garage in many ways.
I use sort 'By Manufacturer' to search for duplicates that I might want/be able to sell.
After sorting I go through the garage scanning for cars that don't have a pink dot (Gift car) and see if there's a Gift duplicate I've won next to it so I can sell the car I've bought to get some money back.
I'm not overly fussed about deleting Gift duplicates, but I don't mind searching for sellable cars to boost my Credit top line.