Us is a movie about an underground lab full of dopplegangers, one to each person, and on a certain day, they break out, and attempt to kill their above ground counterpart, as they've been trapped mimicking the actions of those above, with only raw rabbits to eat, no sunlight, and nobody to care for them. All of them save for one have no knowledge of the english language or customs, and there's a throughline in the movie about being slightly out of sync with the original person. The situation is described as two bodies sharing the same soul, and the dopplegangers often shriek animalistically when murdering someone or during an intense moment. I saw this movie and though it was fascinating, so here's my idea to make it in a normal round of CM.
The event takes place on ice colony, the round goes normallly with the marines landing and being informed of a small CLF prsence nearby. The cryobay isn't awakening any more marines, but there suspiciously aren't any announcements pertaining to this The marines don't know why the CLF here or what they want, but they go to wipe them out because fuck terrorists. While the fighting is going on, each and every human on the almayer at round start and until first drop gets an exact copy of themselves with some working joe esque jumpsuits and a pair of shears, with their two objectives being to kill their doppleganger, and then to take over the almayer. The marine dopplegangers would be gathered up into squads just like the originals to emulate how the tethered in the movie stayed together as a family unit to kill their original family selves.
After the short CLF force is killed and presumably either the CL or IOs investigate what they were doing, they find documents pertaining to a hidden underground lab where some HIGHLY CLASSIFIED documents are laid out with either a USCM or WY logo on it. At around this time, the dropships become locked and a blizzard of some kind comes through the area, obscuring view, it's up to the marines to find out what's going on and survive the replacement. They'll have greater firepower, but bad visibility, and the dopplegangers for the sake of fairness get a tracker to their original self. They are allowed to kill others if they get in the way but their prime goal is getting rid of their original. The shears will have strong AP and delimb properties, to give them a chance to kill the marines with the added horror element of watching as your limbs get torn off in the middle of a blizzard seperated from your squad. While the originals can talk, the tethered can't, but they can communicate with body language and mess with their targets before killing them.
If the dopplegangers somehow take hold and kill all of the marines on the planet, the landing zones will unlock and a doppleganger of the CO will send them all up. The remaining dopplegangers who haven't killed their doubles will do so, and if this all succeeds the dopplegangers will win. Like the movie, only the leader among them can talk, the CO's doppleganger, as to how this happened, if you've watched the movie you will know how, for those that haven't, the "real" CO has some explaining to do about top secret government facilities that they just happened to visit.
I just kinda brainstormed this idea now, it probably couldnt work as well as it does in my mind, and im sure there would be lots of issues to work out, but i can dream.