A double negative is a statement containing two negative words. It is not part of standard English, and its use should be avoided.
If two negatives are used in one sentence, the opposite meaning may be conveyed. In many British, American, and other dialects, two or more negatives can be used with a single negative meaning.
Example:
However, English and American users perceive the double negative form as rustic, uneducated, and nonstandard.
The most frequently used negative words are no, not, nothing, never, none, no one, nowhere, neither, and nobody. There are some words which have a negative element in their meanings although they contain no overly negative affix. These words are: hardly, scarcely, barely, etc.
There are justifiable uses of two negative words in a sentence
Some negative words can be used emphatically.
The above sentence uses double negatives emphatically: "I have to go their last concert."
Some can be used to intend a positive or lukewarm meaning: "I was happy about leaving work early."