Given the cars are seeded (e.g. f0660f), simply importing the top n different seeds in the world pool would be enough. Right now the system is treating everyone's copies of f0660f as if they are different cars, but they are not.
And right now the setup appears to be such that you'll end up destroying all diversity from the gene pool as soon as you have more than a few users.
At least with lots of players, as pubnub said, some of those identically named cars were actually different generations and different cars, but were cousins or otherwise related from a common ancester.
One recommendation would be to incorporate any mutations into the ID, so if it started f0660f and mutated it would be f0660f0b or something, to track parentage (And in fact it would be cool if crossovers were tracked in the ID too, a bit like the Russian patronym or spanish matronym)