Well, two things: Apple doesn't have double shot keys on any of their products (well, maybe on the original Mac 128K and others that used the original Mac keyoard). Also, a double shot wouldn't wear down to a smudge. One of the pics I linked to has a great cross-section of double shot keys, where you can see that if it wears down, you'd still have the same key cap. Doesn't matter if you grind it down with sandpaper. ABS keys do wear down. They'll wear down slower if they're laser engraved, but they'll wear down.