Other mental content is expressible propositionally, whereas to duplicate someone's qualia you have to literally duplicate their brainstate (or at least the functional equivalent of their brainstate).

To duplicate something, you of course have to duplicate it. That isn't really the point. What you are tacitly assuming is that qualia cannot be communicated, know or understood without duplication. ie, there is something special about them in that regard. That is how that approach fails as a dissolution. Rather than showing there is nothing special about qualia, it assumes there is something special.

Other mental content is expressible propositionally, whereas to duplicate someone's qualia you have to literally duplicate their brainstate (or at least the functional equivalent of their brainstate).

I don't know about yours, but my qualia don't look like neural activity.