A double negitive is poor grammer - perhaps Schultz was having Linus speak in a ‘smart grown-up’ way, while still making a mistake that a child would make.
One of the axioms of communication is that we are not deliberately wasting each other’s time. So if I use extra words or a double negative when a positive would work, then you can rightly assume there is extra meaning in there. Another example can be found in professional references. If you ask me if Steve is a good worker, and I say he’s a great family man who is fun to have around, I’m answering your question by not answering your question.
My spelling is well below par (I actually have a learning disability - yes, I know I should cut and past to Word to spell check - usually I do), and as I said, my post was a wild ass guess and perhaps not for GQ. I’m glad that other posters were able to answer the OP.
I still think that Schultz tried to make Linus sound like a little kid trying to talk grown up. Others are free to disagree.
Because there is a subtle difference in meaning. It’s not like drinking diluted root beer at all. “Not unlike” implies a similarity, but not a correspondence. Another way of saying it is “it’s sort of like drinking diluted root beer,” which is difference in nuance.
Litotes is a form of understatement. It is often used by people who are trying to avoid an outright statement. To say that X is “not unlike” Y implies that it’s very similar, but the speaker doesn’t want to commit himself to a statement that it’s exactly the same.
Actually, litotes carries a slight and specialized connotation, a nuanced distinction that is often pf value in conveying a precise meaning. Compare “He was willing to go” with “He was not unwilling to go”; do you see the slight reluctance overcome in the latter?
Litotes is a less imprecise way of conveying these fine nuances than a direct phrasing.