Any biplane flying wings after 1914?
OK, I found one, from a friend's suggestion about hang gliders.  The late 1970's UFM Easy Riser, initially a go-kart engine strapped to a foot-launched hang glider, the "first ultralight."
But this is even more Octave Chanutesque than the 1912 Dunne!  Are there any, shall we say, nonprimitive models?  If not, why?