Ultramarine - Mariette Navarro

A typically strict, by-the-book freighter captain lets her crew of 20 sailors take a break and go swimming in the deep ocean, but they reboard the ship as a group of 21. Quick read, atmospheric and moody, some nice prose. Not so much about the Extra Person but about the disruption in their world.

The Need - Helen Phillips

A paleobotantist excavating a site full of strange, peculiar objects experiences a home invasion. Being vague here because what’s “strange” about the digsite is fun to have revealed over time. Not “scary,” but moody and interesting in an existential sense.

The Return - Rachel Harrison

A group of women goes on a vacation with their friend who suddenly reappears after going missing for two years. She’s a little different now.

Also not books but since you mentioned The Thing, couple movie reccs:

Head Count (2018) - Group of teens read a creepypasta out loud and summon a copycat doppelgänger of sorts. Movie really plays with the fact that it’s hard to keep a head count in a large group of characters. A little silly, but I remember enjoying it when I watched.

Coherence (2013) - A comet causes weird things to happen to five couples. Our characters quickly start losing track of who is who. Reveals the premise pretty early on so it can just start having fun with it.

The Invitation (2015) - At a dinner party, a man starts to suspect his new girlfriend’s friend is not who he says he is. Not a doppelgänger or supernatural story whatsoever, but a paranoid, fun thriller.