Most guppies are inbred...thats how they breed color strains. BUT most breeders cull out the disfigured, weak ones...and most lfs check out the stock before they get them to sell.
Its horrible to see that in a tank though. One of the lfs that I go to buys local stock and always has several different tanks containing the same species for different prices...either because the fish are younger or lower grade...or older and a higher grade.