The phrase "double negative" is not itself an error of any sort, but it does name a construction considered illiterate or narrowly dialectal. Such a construction employs two negatives to express a single negation. Illiterate or careless speech abounds with such expressions as "can't hardly," "haven't scarcely," "can't scarcely." Such double negatives have been allowable in past centuries, but they are now out of style and unacceptable. You are not likely to say "I didn't get no money" or "I haven't seen nobody," but you should be careful to avoid using not with such negative words as no, but, nor, only, hardly, barely, scarcely, and except: "I did not have but five hours' sleep." "You can't help but love that child."
Dictionary of problem words and expressions. Harry Shaw. 1975.