To be completely honest, this is about as broad a theme as one could ask for. This isn't a bad thing, but I'd be surprised if it threw too much of a wrench into anyone's existing plans. I mean, a seven day jam is hard enough, I suppose, without an overly restrictive theme, so it works out. You could literally take any idea you wanted, tilt the lens ever so slightly to fit the theme, and you'd be fine. We tend to fixate on duality as having to do with binaries and the number two, but that's not ultimately what the word in its fullness refers to, having to do more with separation and separateness between discrete ideas or entities - they don't have to be opposites, and there doesn't even have to be two of them, since any act of "splitting" is a manifestation of duality, so if one thing splits in two once, that's duality, but then if one of those halves splits into two again, that's also duality, resulting in the number three, and if all of them split a third time, well, that's Numberwang!
To put it another way, non-duality is a state of unity - not multiplicity. Three, five, six, or five thousand are not examples of non-duality. A non-dualistic view of the universe, for instance, emphasizes the illusory nature of the perceived separation between all things.
Of course, even if you were to stick to binaries, opposites, strict dichotomies, and/or the number two, you're still unlikely to ever have a shortage of possibilities and options, because almost everything comes in pairs and almost everything comes as part of a dichotomy. Did you want to make an SMT clone / tribute? You've got duality baked into that whole franchise. Does your game take place in a hierarchical society? There you go. Sure, the good and evil dichotomy is everywhere and might feel a bit cliché, but at the end of the day it's those things for a reason - the conflict is immediately relatable and has clear, identifiable, and real stakes that are pretty much always relevant to the human condition. And there is no one way you have to go about it. Twin Peaks and The Lord of the Rings are both about that same duality, and one is among the greatest and most unique sagas known to man ... and the other is Lord of the Rings.
Anyways, despite the above spiel ... I probably won't even be able to submit a game for this, because I am WAY too mentally ill, old, and tired to do jams shorter than a month long (at least in my particular case, this sort of thing just doesn't fly anymore, sadly). Still, I am super excited to see what everyone comes up with, and will probably spend the week working on a game just to ride the vibe, so to speak.