The way dualists explain it is what if there was a magical copy of you that also can't think. It would go about its existence behaving like you through contrivance, but since it can't think, it's actually just a biological coincidence of random molecules appearing to be you and acting like you.
This is supposed to disprove the materialist conception of consciousness, because the claim is that if materialism were true, there would be no distinction between you and a coincidental copy that's exactly like you except for your mental states