When a Doppelganger (or Copy Cat from the Vampires expansion) look at a role, their card now takes on the role they looked at. So, if they are swapped by a troublemaker, for example, the person who ends up with the Doppelganger card is the role the Doppelganger looked at.
Further, the Paranormal Investigator becomes any non-Villager role they look at and must then stop looking at cards.
In neither case are cards swapped.
My question is, what happens when the Paranormal Investigator looks at a Doppelganger who has looked at a card on the Werewolf (or Vampire) team?
First, if the Doppelganger is the first card they look at, do they assume that the Doppelganger is a Villager role and then look at a second card? I assume so. That's the only thing that seems fair.
Second, does the Paranormal Investigator become the non Villager role that the Doppelganger looked at?