It is hinted at by the emperor himself (though perhaps not clearly explained). This means that both the emperor from RotJ and senator Palpatine would have been clones. But considering how busy the movie is, I'm not surprised many people missed it. At that point I think the majority of the audience is tuned out or only paying half attention.
Unnecessary? Absolutely. But the 'real trilogy' was already wrecked by the prequels, in which Anakin is an unlikable a-hole who murders children... unless you simply ignore those movies happened, in which case the original trilogy remains a timeless classic.
Bladerunner is also still a classic movie, despite the recent sequel now deciding that Deckard was a replicant all along, only because it was a popular fan theory (there's nothing in the original movie to indicate that this was the case).
John Carpenter's the Thing is also still an awesome movie, despite the awful CGI-filled prequel that we got a few years ago (which also introduced a lot of plot holes).
Those things don't affect me that much.