The Martian Dopplegangers are the primary antagonists of the obscure but well-liked 1962 feature film, The Day Mars Invaded Earth.
Slowly, by creating duplicates of Earth humans in key positions and locations, they enabled the true Martians (never seen or shown) to take over the planet almost without firing a shot. The story focuses on the family of one scientist, a man capable of understanding and perhaps turning back the invasion. But from the start, his own brother is compromised, and it seems that even he had been replaced prior to this, his new replacement having full knowledge of the invasion, which the deep cover original duplicate did not, unwittingly gathering information the invaders could use. Despite cleverness and fierce resistance, the Martian planners and their pawns have done too good a job in setting everything up. In the movie's ending, shocking for its era, the scientist, his wife, their teenaged daughter and young son all end up dead and replaced. The Martian dopplegangers leave no room for error; once a Human is duplicated and the original confronted with their doppleganger, they are promptly burned to ashes, leaving only smoking ashes and a burnt outline like that left by an atom bomb's blast.
No Martian ships, weapons, or even the Martians themselves are seen in the film, which relies on an atmosphere of Cold War paranoia to create its terror. The dopplegangers one and only depicted power seemed to be causing a Human to combust, which causes them to become briefly transparent; they could perhaps also become intangible, but this could also have been superhuman speed at work. Their ability to simulate emotions is decent, and while far from perfect, is enough to not cause suspicion in and of itself. It is also not made clear whether these dopplegangers are altered Martians of some kind, or merely empty-shell Human duplicates. While the duplicates do waste no time in killing their template Humans, they do not need to do this to be created, and the process by which they are made is never shown. Notably, unlike the similar Pod People (who also claim to lack emotions) from the various Body Snatcher films, they do not go out of their way to taunt or gloat about their victory's inevitability, and in fact reveal almost nothing those fighting them can use against their invasion. In the film, neither protagonist nor antagonist is forced to carry the Idiot Ball. The Martian Dopplegangers win out not because of their targets' foolishness or gifts from the plot; they do so because of better technology and preparedness.