Doppelganger (commonly misspelled Doppleganger, even in official books) is a word borrowed from German (Doppelgänger) that literally means 'double-walker'; in myths, it is a paranormal double of a normal person.
Doppelgangers appear in many mythologies across the world, and usually are seen as a bad omen or a punishment on someone. In some traditions, seeing the doppelganger of a friend or relative could be taken a warning of illness or death to come. Grim times indeed.
Because mythology is a free idea bucket for many settings, doppelgangers have appeared in various forms in nearly every franchise possible in some form.
Dungeons & Dragons
In Dungeons & Dragons, doppelgangers have a long history. Described as resembling almost elf-like androgynous grey-skinned beings, doppelgangers are portrayed as skilled shapeshifters with fundamentally lazy temperaments; believed to have originated from some evil wizard's experiments to create the perfect assassin and puppet ruler, they have since gone wild and use their skills to try and replace powerful individuals so they can lead lives of luxury. Consequently, they still tend to end up employed as assassins and "veils" for evil wizards and similar types.
Still, unlike many D&D monsters, doppelgangers aren't inherently, or even commonly evil, just lazy, empathy-impaired jerks. (X5 brings in the Mujina, who runs the exact same grift.) In AD&D, there are several kinds of doppelganger; the most notable is the Greater Doppelganger, which can absorb all of a person's memories by eating their brain.
Ravenloft is home to the creatively named Ravenloft Doppelgangers, renamed Dread Doppelgangers in 3e, which are more malicious, have far more refined mind-reading powers and can use magical illusions to further supplement their disguises. But unlike normal doppelgangers who can theoretically live outside of normal society, they cannot reproduce unless they mate with a humanoid (typically as a male, as a pregnant Dread Doppelganger cannot shapeshift). They hail from the domain of Peridon, whose Darklord is himself a Dread Doppelganger.
In Eberron, doppelgangers are believed to be the ancestors of the Changeling race; 5e retconned that they are instead the descendants of changelings, being the result of Daelkyr fleshcrafting to upgrade changelings to create the perfect assassins and saboteurs. They form the leadership of the Tyrants, a gang that controls much of the prostitution in Sharn. How a gang that's mostly legal enterprise wound up being called the Tyrants is anyone's guess.
A Half-Doppelganger template appears in Dragon Magazine #313.
Half-Doppelganger (Dragon #313)
|Dungeons & Dragons 4th Edition Races|
|Player's Handbook 1:||Dragonborn - Dwarf - Eladrin - Elf |
Half-Elf - Halfling - Human - Tiefling
|Player's Handbook 2:||Deva - Gnome - Goliath - Half-Orc - Shifter|
|Player's Handbook 3:||Githzerai - Minotaur - Shardmind - Wilden|
|Monster Manual 1:||Bugbear - Doppelganger - Githyanki |
Goblin - Hobgoblin - Kobold - Orc
|Monster Manual 2:||Bullywug - Duergar - Kenku|
|Dragon Magazine:||Gnoll - Shadar-kai|
|Heroes of Shadow:||Revenant - Shade - Vryloka|
|Heroes of the Feywild||Hamadryad - Pixie - Satyr|
|Eberron's Player's Guide:||Changeling - Kalashtar - Warforged|
|The Manual of the Planes:||Bladeling|
|Dark Sun Campaign Setting:||Mul - Thri-kreen|
|Forgotten Realms Player's Guide:||Drow - Genasi|