That’s assuming a literal translation is preferred, something discussed in DuoLingo from time to time. I picked this off the top of a Google search: ¿Por que no se puede traducir literalmente frases hechas? If you translate it literally word for word, it comes out: For what no itself can to translate literally phrases made? That doesn’t make any sense at all. A more reasonable translation is: Why are idioms not translated literally? But if you do that, the only word translated literally is, ironically, the word “literally”.
It isn't really a double negative. You can say, "No, that's not true; I never had it, and I don't have it now. Four negatives, but perfectly grammatical. "Double negative refers to a statement like, "I don't have no soap" or She did not never do it", which are not acceptable in English.
Actually, in order to learn a language one has to go over material more than once. You do not remember everything presented the first time you encounter it. For example, I've done the WHOLE dates and times lessons twice and am still shaky on the spellings of one or two of the days of the week, of several of the months, and a couple of the seasons. The Romans had a saying, "Repetitio mater memoriae." It means "Repetition (is) the mother of memory." The Romans were right. Having taught a foreign language - not Spanish! - in the public schools, I can testify that repetition is essential to students' learning the material. This is the reason why Duolingo asks you to repeat material to "strengthen" a lesson you have already completed and why it throws in sentences that do not pertain to the particular topic you are learning in whatever the current lesson happens to be..
Well, if you look into the context, you can infer that it would just be "no, it's not fair". If you really want to write that someone is not fair/just without having much context to go off of, I would suggest you place a personal pronoun in front of the "es" to get that across clearly. It's all about context, really. I don't know if that made sense, sorry. I'm a native English speaker, but explaining is not really my thing.
In "no, no est justo", the first Spanish "no" translates to the "no" in the English translation. The second Spanish "no" translates to the "not" in the English translation. You have used up all the no's in the Spanish sentence, and there are no more Spanish no's left left to translate to another "no" in the English translation.
Another accepted translation:
No, it is not just!
(April 16, 2020)
BTW, this is NOT a double negative in English. Here are some examples of double negatives: