I remember that question, and I had the same confusion you did. I guess this is "Duo" as in "Duobot", as in "Duolingo", which I guess makes the sentence kind of funny... Unfortunately, I didn't figure that out for a while and viewed this as "das Duo". I whined about it in a "still think you're right?" comment... but I guess I wasn't all that right :-)
I basically agree with YOLO. But it is more complex. This one is worth reading! http://www.heinrich-tischner.de/22-sp/9sp-ecke/fragen/210/2012/wissen.htm
I put 'Duo knows every answer' as the translation, and was told it was wrong. In at least one context, 'Duo knows all the answers' (the given correct answer) means exactly the same thing.
Is there anything subtle in the original German that makes my translation wrong and the given translation right?
I never would have figured this one out. I tried "The pair knows all the answers." Didn't work. I don't like this one at all. And why is "the" not allowed for first noun in this sentence but not the second? I know it would be improper in English, but why not allow an article for both nouns? It seems like articles in the translations are allowed or disallowed arbitrarily, and it's incredibly frustrating.