Doppelgänger

Talk about being your own worst enemy.

—Lara Croft after she sends a mutant copy of herself into a lava pit

Twin. Doppelgänger, with an umlaut and a majuscule, is the German word for a ghostly double of a living person or more commonly simply a word for someone looking exactly like someone else, i.e. an impostor or double. It means "double (walker)", in the original. It is often said that one who sees their own Doppelgänger will die soon.

Traditionally, Doppelgängers have strange, supernatural origins, unlike twins which usually have more natural explanations. A Mirror Universe, Cloning, alien intervention, and magic are all possible sources of a Doppelgänger. For purposes of clarity, this index includes both mundane and supernatural impersonators of a character.

For twins related by blood, see Twin Tropes. See also Disguise Tropes. If a good character has an evil Doppelgänger, the Doppelgänger is an Evil Twin.

Specific types of Doppelgängers include:

Alternate Self
Alternate Universe -- self explanatory.
Artificial Human -- Clones and bioroids are one common way of making duplicates.
Body Snatcher -- Doppelgangers created by bodily possession.
Changeling Tale -- when The Fair Folk abduct children, they often leave behind a "changeling".
Costume Copycat -- When a hero is impersonated by someone in a nearly identical costume.
Criminal Doppelganger -- When the person's double is also coincidentally a wanted criminal.
Dead Person Impersonation -- When one person takes over another's identity.
Easy Impersonation -- When a hero is impersonated by someone in a not-even-similar costume.
Evil Counterpart - A character (not related to the hero) who is very similar, but For Want of a Nail turned evil.
Evil Knockoff - when a robotic duplicate or clone is made of a hero with the purpose of destroying them.
Evil Twin -- When a character has an evil sibling or alternate reality duplicate.
Face Stealer -- When a villain has to harm another character in some way to look like them.
Freaky Friday Flip -- When two people switch bodies, the result is remarkably doppelganger-like.
Humanshifting --A type of Shape Shifter that turns into humans.
Identical Grandson -- Superpowerful Genetics is so strong, it can give you the original person back! (Genetic Memory also helps.)
Identical Stranger -- For some reason, the gene sequencing fairy took the day off and you have an unrelated twin.
Jekyll and Hyde -- When the person is their own doppelganger.
Legendary Impostor
Literal Split Personality
Mirror Self
My Sibling Will Livethrough Me -- When a sibling takes over a person's identity.
Opposite Sex Clone
Our Clones Are Identical
Replicant Snatching -- Or when a robot/alien peels off the original's skin and wears it.
Robot Me -- When the doppleganger is a robot.
- Mobile Suit Human -- When the robot duplicate is piloted by a tiny alien.* Separated At Birth -- two (often but not always identical) twins are, well, separated at birth.
Shapeshifting -- Some doppelgangers can do it by shape shifting.
Split At Birth
Surgical Impersonation -- When the doppelganger used Magic Plastic Surgery to impersonate someone.

Related tropes:

Actually a Doombot -- When the person is revealed to be a robotic duplicate.
Ambiguous Clone Ending -- Did the original kill the clone, or is the clone the survivor?
Becoming the Mask -- What happens to many doppelgangers.
Body Double
Counterpart Combat Coordination -- fight your doppelgangers.
Creepy Child -- often come in twos, or more.
Distaff Counterpart: Very similar to the original character, just the opposite gender. Often a rival or villain.
Doppelganger Replacement Love Interest -- That Love Interest that just died? They have a duplicate in the real world that wants to shack up with you.
Emergency Impersonation -- An impersonator tries to fill in for someone else and Hilarity Ensues.
Expendable Clone -- The tendency to see these doubles as highly expendable, if not abominations to be killed.
Gendered Outfit -- When an Opposite Sex Clone, Distaff Counterpart, or Gender Bender wears an opposite-gender version of an established outfit.
Glamour Failure -- Not all doppelgangers can hide their tells.
I Say What I Say -- Doppelgangers speaking in unison.
Identity Impersonator -- A ploy heroes use to throw off people are snooping their Secret Identity.
Impostor Exposing Test -- A test to identify an alien or supernatural impostor.
Imposter Forgot One Detail -- An impersonator fails to to be convincing.
Kill and Replace
Mangled Catchphrase -- One person imitates another but screws up the first person's Catch Phrase.
Mes a Crowd -- when someone tries to clone themselves to do their chores.
Mirror Match -- When a character fights themselves (or a reasonably close match) in a video game.
Mirror Morality Machine -- A character is made into their moral opposite thanks to Phlebotinum.
Mirror Universe -- Home to many morality opposite versions of a cast.
Mismatched Eyes -- a character with Odd Eyes or Eyes of Gold may be a doppleganger of some sort.
Mistaken for An Imposter
Opening a Can of Clones -- A story with clones, especially when used to revive dead characters or undo plot, makes the events lose punch.
Other Me Annoys Me
Prince and Pauper -- Two people switch lives.
The Psycho Rangers -- An entire team of doppelgangers to the Five Man Band.
Screw Yourself -- Yes, there is Power Perversion Potential to be had here (also Rule Thirty Four).
Spot the Imposter -- When the friends of the impersonated have to figure out who the spy is.
Teleporter Accident -- One surefire way to create a duplicate, evil or otherwise.
Twinmaker -- A clone is made to replace a dead original.
Villains Blend in Better -- Most villainous doppelgangers easily impersonate their target.
You Are Who You Eat -- A non-human shapeshifts into the form of the person they've eaten.