No, it marked "Is it a plus or is it a minus?" as wrong too. I think you had to say "Is it a plus or a minus?" - making it a question clearly because of the word order in English but while avoiding directly repeating the extra phrase that's included in French. Duolingo wants this answered very particularly!
Ok, I tried again. The problem was not the duplication but "it is" instead of "is it"... And the duplication is ok. These ladies and gentlemen, the great thinking heads of DL, you who are so scrupulously attentive to rigorous translations, if you want us to translate "is it", start by proposing "Est-ce" in your exercise. "Est-ce = Is it" "C'est = it is". Too difficult to understand for your big brains? Or just the pleasure of trapping people with gross errors? . .
In standard English, both in writing and in speech, you ask questions with a Verb-Subject inversion.
In French, the Verb-Subject inversion is the more formal interrogative construction (reserved for formal writing).
So you have to try and be careful because the question mark and inflection will tell you that a statement is a question which needs to be translated to a standard English question.
Now, there are many questions in this course, some of them with an inversion, some with "est-ce que" and some in the statement form like this one. In conversations, probably 80% of the questions you will hear will be in the latter form. Therefore, you need to recognize a question when you see one (question mark) and when you hear one (the woman's audio is perfect here, with her raising intonation at the end).
Ok. Thank you so much once again for your very clear explanations. However in my opinion it is not enough to explain the DL's reject. You're right, we must be careful to detect an interrogative form. But I stay on my first idea : if DL allows itself to write a question without inversion, he has to accept an answer without inversion too. Or better, it would be well to rewrite the question with inversion, to avoid this kind of misunderstanding which he seems to love. . . Thanks again for your availability.