But they have no information about what I actually ate for breakfast! What is the "evidence" that allows them to be distinguished?

I have evidence that Mitchell1 thinks there are problems with the MWI. My duplicate has evidence that Mitchell2 thinks there are problems with the MWI. Mitchell1 and Mitchell2 are not identical, so me and my duplicate have different pieces of evidence. Of course, in this case, neither of us knows (or even believes) that we have different pieces of evidence, but that is compatible with us in fact having different evidence. In the Boltzmann brain case, however, I actually know that I have evidence that my Boltzmann brain duplicate does not, so the evidential distinguishability is even more stark.

This whole line of thought arises from a failure to distinguish clearly between a thing, and your concept of the thing, and the different roles they play in belief. Concepts are in the head, things are not, and your knowledge is a lot less than you think it is.

I don't think I'm failing to distinguish between these. Our mental representations involve concepts, but they are not (generally) representations of concepts. My beliefs about Obama involve my concept of Obama, but they are not (in general) about my concept of Obama. They are about Obama, the actual person in the external world. When I talk of the content of a representation, I'm not talking about what the representation is built out of, I'm talking about what the representation is about. Also, I'm pretty sure you are using the word "knowledge" in an extremely non-standard way (see my comment below).