It is not true, as some assert, that double negatives are always wrong; but the pattern in formal speech and writing is that two negatives equal a mild positive: “he is a not untalented guitarist” means he has some talent. In informal speech, however, double negatives are intended as negatives: “he ain’t got no talent” means he is a lousy musician. People are rarely confused about the meaning of either pattern, but you do need to take your audience into account when deciding which pattern to follow.
One of the funniest uses of the literary double negative is Douglas Adams’ description of a machine dispensing “a substance almost, but not quite, entirely unlike tea.”
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