It's funny how many people simply take it as a matter of course that a second negative in a sentence negates the first one, rather than intensifying it. That interpretation of a second negative seems unique to modern, pedantic English. Consider Bede:
"Ne con ic noht singan!" for thon ic naht singan ne cuthe.
Four negatives piled up in that one little sentence of Old English. And no one thought Bede was saying the guy could sing, but in a funny way.