how would you GM something like a "shapeshifters impersonating the PCs" situation?

[Plane]

For example like the "Migoi" in the Sanctuary TV series episodes "Kush" and "Icebreaker".

Basically each PC knows "I'm not a shapeshifter" but they don't know if who they're interacting with is another PC or a shapeshifter.

This type of thing seems easier if it's just shapeshifters impersonating NPCs since the GM roleplays whoever you interact with.

There is a clear appeal-to-metagaming problem when it comes to impersonating other PCs though because (especially with in-person gamine) you know whether the player or GM are speaking.

In situations like this, would it make sense for the GM to request the player to not just play their own character, but also their shapeshifter NPC doppleganger, so that other PCs can't figure out who is who?

The PC shouldn't really be able to control the NPC's actions totally though, so the GM should probably tell them roughly what to do, and be passing notes and stuff.

I would think that might be easier w/ online gaming since the timing/frequency of note-passing might give away which version is the NPC impersonator in IRL games.