I think I accidentally duped someone once lol.
Well, it was his fault. I was selling a bricked Wii (I updated to the latest firmware and it bricked the Wii since it was pirated) just to get some of the money back. Nobody was buying it, so I kept lowering the prices until someone came by and bought it. A few days laters, we receive a very angry e-mail from him telling us that the Wii didn't work. He tried to get a refund, but couldn't, since the page where I was selling the Wii was very clear about it being bricked.
Basically, he bought what he didn't want and it was his fault, but I think it does kinda count as a dupe for him.