Doppelgänger (The Twilight Zone)

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

Biology
Doppelgängers appear exactly 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 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 as Millicent was waiting in a bus depot, en route to Cortland for a new job.

Appearances

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