A very pompous linguist was
giving a talk at Columbia and noted that there were
languages in which a double negative meant a positive
(standard English, for example: “I am not going to not
speak” = “I am going to speak”) and languages in which
a double negative is a stronger negative (standard
French and Italian, for example; or non-standard
English: “You ain’t got no class”). But, says he, articu-
lating what he imagined was a universal of grammar,
“There are no languages in which a double positive is a
negative.” Pause. Silence. Then came a loud and
knowing sneer from the back of the room: “Yeah,
yeah.”

http://www.prickly-paradigm.com/paradigm4.pdf