Tl;Dr: "Wave-particle duality" is not the notion that matter is "sometimes" a particle and "sometimes" a wave. It is, at all times, its own separate category of thing, for which "particle" and "wave" are just metaphors that approximate its behavior.
One metaphor usually comes closer than the other depending on what system you're looking at, but it's never changing back and forth between some "particle state" and "wave state".
One metaphor usually comes closer than the other depending on what system you're looking at, but it's never changing back and forth between some "particle state" and "wave state".