Is 必要なくない the same as 必要なくはない?
I know that なくはない is a double negative and it means "It's not that... [sth isn't]..." However, I asked my Japanese language partner about the difference and she can't explain it well, but she says that they are different. I'm confused because I can't find any entry on なくない.
The difference is the same as that between 赤くない and 赤くはない, etc. As far as this は is concerned, there is nothing specific to double negation. は in a negative sentence can be an explicit contrast marker, but sometimes it's nothing more than a particle you use unconditionally in negative sentences.
Why is the topic marker often used in negative statements (ではない, ～とは思わない)?
So how did the は come to be used anywhere? This is probably part of an historical process of trivialization and loss of meaning. It seems to me that originally, the pattern ～ではない was used to only mark a strong contrast of the entire predication to something else (e.g. something that was said or implied before by someone else), but later became more and more popularized until what originally was a contrast marker became an almost necessary feature of the negative form.
は in 必要なくはない can carry a strong contrastive meaning (similarly to 必要ではある which is always strongly contrastive).
値段はとても高いが、必要なくはない。
It's very expensive, but it doesn't mean it's unnecessary.
(は is mandatory here)値段はとても高いが、必要ではある。
It's very expensive, but it is necessary.
When this contrastive meaning seems unimportant, you can think it's just a common and natural way to say "It's not unnecessary". In such cases, it's basically interchangeable with 必要なくない. Still, 必要なくない sounds colloquial, and you should use 必要なくはない in formal settings. I suppose most textbooks use なくはない almost unconditionally.
