From a linguistics lecture:
Lecturer: In general, when it comes to negation, languages, or more precisely dialects and registers of languages, tend to fall into two groups. On the one hand we find those languages which employ more than one negative to emphasise negativity. And on the other hand we find those languages, dialects and registers that follow the rules of logic so that a second negative cancels out the first. So we have languages where two positives make a positive, two negatives make a negative, and languages where two negatives make a positive. But we have never come across a language where two positives make a negative. Ha, ha, ha.
From the back of the room comes a voice: Yea, right.