Dupighosts are ghost-like creatures that can take on the appearance, voice and abilities of other people. They first appear in Paper Mario. To attack, Duplighosts can either fly into Mario, or turn into his current partner; in this guise, the Duplighost gains that partner's attacks and physical properties, e.g. a Duplighost turned into Watt will damage Mario or his party members if they make contact with some moves. However, those attacks are weaker than those of the actual partner.
The name "Duplighost" is a portmanteau of the words "duplicate" and "ghost."
In Paper Mario, Duplighosts appear most often during Chapter 7, in the Crystal Palace. They use the mirrors in the palace to imitate other objects and characters in order to confuse Mario.
Several instances of a Duplighost impersonating someone occur throughout the game. On the path to the Crystal Palace, one Duplighost pretends to be Kooper; Mario must hammer the impostor, then defeat the Duplighost in battle. In the Crystal Palace, four Duplighosts appear and pretend to be Bombette when Mario uses her to destroy one of the walls. The fakes can be distinguished by the way they speak, such as using more hearts or more exclamation points. Hitting the impostors with Mario's hammer will expose them, with all four running off once they are revealed; if the real Bombette is hammered, Mario must fight the four Duplighosts. Another instance occurs within the Crystal Palace when four Duplighosts attempt to impersonate Kooper, but instead disguise themselves as Luigi, Goompa, Koopa Koot, and Kolorado. As before, Mario can drive off the obvious fakes by hammering them, and if the player opts to hit Kooper instead, the four Duplighosts must be battled.
Another Duplighost appears later on in Bowser's Castle, trying to trick Mario into believing he is Princess Peach. Mario can break the Duplighost's disguise by hammering the fake princess repeatedly, leading to a battle where he must defeat four of them.
A blue Duplighost named Lee is one of the students at the Toad Town Dojo, and can be fought as an optional boss to obtain the Second-Degree Card.
Paper Mario: The Thousand-Year Door
Paper Mario: The Thousand-Year Door has only one Duplighost, Doopliss, who stays in the Creepy Steeple. Many of the Twilighters refer to him as a demon, since they have never seen him in person. Doopliss appears as an enemy of Mario several times throughout the game, starting with the events of Chapter 4.
Doopliss causes the members of Twilight Town to become pigs whenever the bell tolls, and demonstrates the special ability to steal another character's body while copying them. Doopliss attempts to steal Mario's identity, even succeeding for a little while, until he is exposed with the help of the real Mario and Vivian. for a little bit. Doopliss later becomes a member of the Shadow Sirens, replacing Vivian. In battle, he has the same powers and abilities as regular Duplighosts, using the same swooping attack and transforming into Mario's partners to use their attacks