Really? I can usually remember if I own a game or not. Most of the time I can tell you what I paid, and where I got it. I don't need to rely on VG Collect for this. The only console that I get a little mixed up on is NES because there's tons of games and I've helped others collect for it, but 99% of the time I'm right without even checking here. As far as the Wii U which doesn't have many games in total, it's one the smallest collections that anyone has by default. Really easy to remember, IMO.
Don't think I've ever double-bought a single game, unintentionally.