Doppelgänger (The Twilight Zone)

The doppelgängers are malevolent counterparts of Humans from a parallel dimension.

Biology
Doppelgängers appear externally identical to Humans, and are presumably extremely close to or exactly the same as Humans in internal physiology. As their dimension is a parallel one that is a copy of the main universe, the doppelgängers are exact duplicates of Earth's inhabitants, and thus have counterparts on Earth. It is possible that they have some supernatural ability that allows them to know what their counterpart's life is like, but this is ultimately unconfirmed.

Culture and society
Little is known of doppelgänger society, which is possibly a reflection of all societies found on Earth. They have been seen dressing in the exact same way as their counterparts, and have been able to blend in on Earth. However, they appear to have knowledge of the nature of their dimension and Earth's, as they know that the only way for them to survive on Earth is to take over the life of their counterpart, also implying that they know exactly what their counterpart's life is like. To accomplish this, they would quietly infiltrate areas where their counterpart is mostly secluded, and act as their counterparts whenever their counterparts aren't present or when no one is looking a them, showing incredible talent in their imitations, enough to fool surrounding Humans, but often run out of sight when their counterpart starts to notice them. Eventually, they will find a way to eliminate their counterpart, and fully take over their lives. They are known to act maliciously, smiling at and taunting their counterparts, but never making any attempt to directly communicate or interact.

Technology
Almost nothing is known of doppelgänger technology, but it is presumably exactly like Earth's, or at least enough to create clothing that appears externally identical to those worn by their counterparts. It is unknown if the dimensional bridges between their dimension and Earth are the result of some technology they created, or some sort of natural phenomenon.

History
At some points in time, some sort of event, whether artificially created or natural, pulled doppelgängers from their native dimension to Earth, where they took over the lives of their counterparts. Sometime in the 1960s, a doppelgänger of a Human woman named Millicent Barnes arrived on Earth. Millicent was waiting in a New York bus depot, en route to Cortland for a new job, and her doppelgänger soon arrived at the same station, where there were few other people present. While Millicent's back was turned, the doppelgänger went to the ticket agent and asked twice when the bus would arrive, as it was running late, also taking Millicent's bag and placing it behind the desk. The ticket agent responded both times to the doppelgänger, and did not notice that there were two of the same people present in the station. When the real Millicent went to the ticket agent to ask the same question, she notices her bag behind the booth and realizes that something was wrong. At the same time, the doppelgänger used the restroom, noticed by the cleaning lady. The real Millicent then goes to the bathroom, and the cleaning lady, thinking that the two beings were one in the same, tells Millicent that it is her second time in the room. Millicent then sees the doppelgänger in the mirror, sitting at the same bench she was just at. The doppelgänger then hides, presumably going outside, just before Millicent could exit the restroom or get anyone else to notice the doppelgänger. Millicent then met another passenger named Paul, and told him about the doppelgänger, but Paul didn't believe her. When the bus arrived, the doppelgänger was quick to get on it, smiling evilly at Millicent just before she could board. Millicent ran back to the station and fainted, allowing the doppelgänger to leave on the bus and take over Millicent's life. Sometime before the 7 AM bus could arrive, Paul's doppelgänger arrived on Earth, getting to the same bus station. After Millicent is taken away by two policemen, assumed to be insane, Paul's doppelgänger enters the building, unnoticed by Paul, and takes his suitcase. Paul notices his missing suitcase and manages to see his doppelgänger running off, chasing him down the street. The doppelgänger kept running, looking back at Paul and smiling malevolently, his identical visage revealed under the street lights. The doppelgänger manages to vanish into the darkness, leaving Paul shocked and confused on the street.

Appearances

 * The Twilight Zone, s01e21, "Mirror Image" (1960)