In my laptop, NOT(NOT P) = P
It's outside of laptops, in the use of English by native speakers of English, that "She's not unattractive" turns out to be an oblique "damning with faint praise" sort of non-compliment about a woman's appearance, definitely not equivalent to saying she's attractive. This actually is a way of dealing with (and when used as in the last example, playing on) a problem that doesn't arise in laptops: vagueness. There is not sharp universal standard of attractive vs. unattractive as regards feminine pulchritude, just as there is not a universal sharp standard for common vs. uncommon in discussing a behavior or social phenomenon (the original matter in this thread which hyper-Aristotelians jumped on).
The gap between what everyone would regard as common, and what everyone would regard as uncommon, the space where folks would quibble with one saying "We'll I've seen a lot of it, I think it's common" and another saying "What? There's hardly any of that. . .now this [other, nearly universal behavior] is common," is what provides the distinction between the natural language use of "not uncommon" and "common": "not uncommon" denies that the phenomenon is what would generally be agreed to be uncommon, while not asserting that it is what would generally be agreed to be common.