Let's Read and also Play Baldur's Gate: Bhaal must be stopped!

[Khay]

So, while my knowledge of AD&D forgotten realms isn't perfect, I'm reasonably certain that Waterdeep isn't actually ruled by dopplegangers at that time, and they usually only hang around in their real forms in their lairs, so it makes a bit of sense that Abdel and Jahira don't recognize them on sight, especially since they tend to be city/dungeon monsters more than forest monsters.

That makes sense. It can be hard to tell whether information is supposed to be a reliable, but on second reading, I suppose this does sound like an example of being incorrect in-universe.

As for the doppelganger monster: When did this thing pop up as a shapeshifting monster? I know the thought of someone showing up and taking your place in society while you are imprisoned or away is not new to literature and film. I know Grimm's Wörterbuch - a dictionary - has defined Doppelgänger as someone who looks exactly like you and takes your place but I don't know of any Grimm's fairytale that uses a Doppelganger. Did D&D just put the stories into a monster?

Probably - possibly taking inspiration from sci-fi as well as fantasy - "Kill/Capture & Replace" was very common in science fiction, and the D&D monsters look rather like "Greys" "Roswell Aliens" etc.

Doppelgangers and changelings have a long history in folklore, yeah. Fiction, too - for some reason the doppelganger invasion plot was a real trend in the 1950s. Think Invasion of the Body Snatchers. D&D mined (and continues to mine) both folklore and popular culture for material, and this is no exception.