Not not:

As you don’t not know, if you use double negatives in English, you’re not unlikely as not to be clear.
It’s tricky to count negs on the fly when an even number of negatives means a positive. But in many other languages (such as Spanish and French), multiple negatives simply accentuate the “not.” In English, using multiple nots for emphasis rather than negation seems to be ‘against the rule”, but Mencken writes here about such usage, longstanding, in English. The WikiPedia entry on double negatives includes a good joke about double positives.