I'm not sure what order you are doing your lessons but if you aren't skipping any and doing them in sequence then you would have been exposed to noix as having two meanings, nut and walnut.
Personally, I always have difficulty with it because it isn't given very often and sounds very indistinct to my ear when rendered by the robot. I'm commenting on this particular instance because, again, I couldn't figure for the life of me what the robot was saying. Not because I haven't heard it before though.
Another lost heart. (sigh)
Go to Google Translate and practice listening to the words repeatedly one after the other.
I'm not sure if it will work for you but then I'm more interested in getting correct answer than spending a lot of time on a word that is fairly infrequent.
As for how to tell them apart by other means just do the following.
Look for a modifier. noix will always have a modifier of some kind. Noir is the modifier for some noun or pronoun.
English has many homonyms which are words that when pronounced correctly sound identical but have very different meaning. We rapidly identify the intended meaning by use of context. Do the same thing with French as much as possible.
Over time you will eventually be able to distinguish sounds that you couldn't identify before. Of course I don't know if you will ever be able to distinguish the Duo robot rendition of noix and noir. I have yet to isolate Duo's take on the words. But like I said I don't much care as long as I can figure it out otherwise.