I know a duvet is a plain comforter, but who wants just a plain cotton white one? I always buy a size up in comforters, that's why the one I had was so long. I do the same with duvets and it makes practically no difference. I have a double bed now with a queen duvet and it still barely even hangs over the end.
I know the trick to putting the covers on and all that, but it doesn't make a difference. Nothing will make me hate them less until they morph into real, patterned, colored, different materials that don't "need" a cover!