Jim likes to give us his interpretations of myths.
What about a Doppleganger? One operating on the level of, say Cat Sith or Eldest Gruff?
Oh, yeah.
Jim explicitly says, IIRC, that "some version" of almost every myth, including modern "urban legend" and overt fantasies like LOTR, Earthsea, &c ... are all true, somewhere in the Dresden'verse.
So a "Doppelganger" -- whether in the older myths, modern urban-legend-y tellings, or overtly D&D-esque -- is almost certainly a real thing in the Dresdenverse; maybe more than one (he did it with Vamps, after all!).
Nice suggestion BTW!