I strongly suspect Duals are not more expensive because of near-indistinguishable fakes that entered the market a long time ago.
Yeah, even if people think their opponent is using fakes the vast majority of the time they won't call the judge. Those early print runs were inconsistent enough that it makes people unsure of their convictions and folks don't want to "be a dick" or whatever so they just let it slide if the cards are fairly close. That said, I wouldn't recommend anyone risk it because I'd be willing to bet that WOTC is going to get draconian over the next few years on folks found to be using a lot of fakes. EDIT: Unless it's at a Legacy/Vintage event. WOTC doesn't give a fuuuuck about those.