Duplighost

Duplighosts are ghost-like creatures that can take on the appearance, voice and abilities of other people. They first appeared in Paper Mario, and a single Duplighost was later featured in the sequel, Paper Mario: The Thousand-Year Door. To attack, Duplighosts can either fly into Mario, or turn into his partner. In partner form, a Duplighost's attacks are weaker, and changing takes up a move, giving Mario a better chance at winning the battle.

Paper Mario
A Duplighost was first seen in Paper Mario at the Toad Town Dojo, where Mario fought a blue-sheeted Duplighost named Lee. More Duplighosts appeared in Chapter 7, in the Crystal Palace, where they used the mirrors in the palace to confuse Mario, as the reflections make it tricky to determine whether a character is the real thing or a Duplighost. Another Duplighost appears 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.

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.

Paper Mario Tattles
In addition to his tattle for Duplighosts that aren't in disguise, Goombario says different things depending on which of Mario's partners is being impersonated by a the Duplighosts he's tattling at the time.
 * Disguised as Goombario: "It's a Duplighost disguised as me. It'll do Headbonk and Tattle. That looks nothing like me. Does it? No! Seriously though, does it? Mario? Hello? Look, I've been working out like crazy trying to get in shape for adventuring. There's just no way I'm that pudgy!"
 * Disguised as Kooper: "It's a Duplighost disguised as Kooper. This impostor almost looks better than the original! Ha! Its disguise is pretty much perfect. Its attack power is exactly the same as the character it's imitating. It's probably not very difficult for this Duplighost to disguise itself as Kooper. After all, our pal is pretty unusual looking. It'll attack us with its shell, just like the regular Kooper. You'll have the best luck beating it if you flip it."
 * Disguised as Bombette: "It's a Duplighost disguised as Bombette. Its disguise is pretty much perfect. Its attack power is exactly the same as Bombette's. You should be careful even though she looks cute."
 * Disguised as Parakarry: "It's a Duplighost disguised as Parakarry. He looks even more intelligent as an enemy. Its disguise is pretty much perfect. Its attack power is exactly the same as Parakarry's. Don't underestimate him, Mario. He looks pretty serious."
 * Disguised as Lady Bow: "It's a Duplighost disguised as Bow. Its disguise is pretty much perfect. Its attack power is exactly the same as Bow's. Look, you've seen her in action. Do you really want to know what a Smack Attack feels like?"
 * Disguised as Watt: "It's a Duplighost disguised as Watt. Its disguise is pretty much perfect. Its attack power is exactly the same as the character it's imitating. You know how Watt is shocking? Same goes for this Duplighost."
 * Disguised as Sushie: "It's a Duplighost disguised as Sushie. Its face looks even grouchier than the real thing! Its disguise is pretty much perfect. Its attack power is exactly the same as Sushie's. I'm glad we have Sushie on our side. She's intimidating! I wonder if this impostor is meddlesome as well..."
 * Disguised as Lakilester: "It's a Duplighost disguised as Lakilester. It's hard to take him seriously. Its disguise is pretty much perfect. Its attack power is exactly the same as Lakilester's. It's tough to dodge his Spiny Flip, so pay attention."

Trivia

 * The name is a pun off of the words "duplicate," which means to make copies of, and "ghost."
 * If the enemy copies Lady Bow, and the player has the Defend Plus Badge on, the enemy will pretty much do nothing to the player.