Around a year ago, I wrote a post deploring the use of negatives in English entitled Notty Tendencies . The main point was the use of negation to avoid being direct, i.e. such as saying such unappetizing food when you mean how bad it is. A year older and a year wiser, I wish to amend my remarks to say that there is an appropriate role for the double negative in English.
Most Latin-based languages expressed the negative by doubling it as in je ne said pas where the ne and pas are both negative markers. In fact, a lone ne in French does not expressive negativity but instead inequality (see http://grammaire.reverso.net/3_1_40_ne_expletif.shtml). By contrast, in English, once is enough. It is acceptable to write I am neither hungry nor thirsty or I am not either hungry or thirsty but not I am not neither hungry nor thirsty. The reason for the rejection of the last one is that it includes two negative markers, not and neither/nor. So, double negatives do not make a negative in English but generally lead to frowns by English teachers and editors.
However, they sometimes create a neutral quality. To demonstrate, look at the following sentences:
a . The mask is not uncomfortable.
b . The girl is not unattractive.
c . The chap is not unpleasant.
In all three cases, there is a double negation, specifically the word not and the prefix un. The justification for the apparent redundancy is the intended meaning. The writer does not wish to say that the mask is a pleasure, the girl is pretty and the man is charming. Instead, the implications are that the mask is tolerable to wear, the girl should be able to find a date for the prom and the man does have some social skills and uses deodorant. In these cases, the doubling most peculiarly creates moderation of the base adjective, i.e., comfortable, attractive and pleasant.
To a native speaker, this subtlety is obvious. However, foreigners can easily misinterpret the intent of the speaker or writer. Learning a foreign language is truly peeling an onion: there is always another layer to grasp and apply. On that note, it would not be unwise to stop, excuse me, advisable, to stop and let you consider the strange case of the double negative (neither with a barking dog nor written by Mr. Arthur Conan Doyle).