I lost my Zelda 1 and 2 cartridges years ago, mom probably sold them at a garage sale after I moved out. So I got those again. I got most of the games available on Animal Crossing, like Punch Out, Excitebike, Donkey Kong.
I only have an NES any more, all my other systems I didn't keep, so I have bought games for other systems that I have had before.
As for the NES, so far from my cartridge collection I have duplicated Teenage Mutant Ninja Turtles and that's it. Mario 3 when it comes out. My Punch Out is the Mike Tyson's edition, so I don't consider it a duplicate.
I don't consider it a "waste", because even if you own it for another/original console you are paying to be able to play it on your Wii. Some people prefer the original feel, while others like the the array of controller options, portability, and convenience. Buying it again simply shows the publisher that at least one person out there (you) still appreciates the work, which helps keep that title alive.
I actually already own all of the titles I have on the VC but I just love being able to play them on the Wii....I guess I am lazy but it is just more accessible if you know what I mean lol...