I'll spoiler the whole thing, in case of players wandering in.

Spoiler: The whole thing
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I would rule that it looks at the body in this case, and so should have been on the scheming guy.

Part of this is because of the bit about having met but don't know. Let's say that a being that can scry meets someone. That someone introduces themself as Larry the Great. In reality, they are Daryl the Mediocre, but they claim to be Larry because he's better than them in every way. Later on, the being that can scry attempts to scry on "Larry the Great". They don't have any idea that Larry the Great is someone else - they clearly intend to scry on Daryl the Mediocre, but they know him as Larry the Great. If they knew him well, they would know he is Daryl and they'd scry on him. If they had only heard of some guy named Larry the Great, then an attempt to scry would actually go to Larry the Great rather than Daryl. But since it's just some person they met, they pretty much take him as who he claims to be, and until they learn something different, for them, he is Larry the Great and that's who a scrying would go to.

Now, it complicates things a bit since the guy they are attempting to scry on is not the guy they met (correct? If they met the Duke post take over, then I think it's clear who they scry on). But my other bit of reasoning is what do they actually associate with the Duke? They don't know his soul - that, for me, would be someone you know well. They know his body. They have an image of who they are trying to scry on, and so the sensor would seek out that image.

Now, this does bring up some other issues. If someone impersonates the Duke through a spell, who does the scry settle on later when trying to look at the Duke? (I'd go with it looks at the Duke because their mental image matches their desire to see the person with that name.) If someone impersonates the Duke through mundane means? (I'd go with it being the person who was pretending to be the Duke, unless they had done such an incredible job with makeup and acting that the person could be introduced to the Duke now and think they had already met the Duke.) So it's definitely not something I would say is the obvious answer, but that's where I'd go.