Legendary Impostor

The protagonist has developed a reputation, and some con artist tries to use it to rip off people who have heard of, but never met, the protagonist. Fortunately, the real deal happens along in time to straighten things out.

Related to Costume Copycat, but not necessarily involving a costume. When the real deal meets the impostor, expect Because I'm Jonesy to occur. See also Fake Ultimate Hero, Famed in Story, Mistaken for Special Guest.

Anime and Manga

 * The 2003 anime adaptation of Fullmetal Alchemist episode "The Other Brothers Elric"
 * The Saiyuki episodes featuring the impostor Sanzo group (whose success is largely aided by how unlike expectations the real Sanzo band is).
 * The Trigun episodes "The $$6,000,000,000 Man" and "Goodbye For Now"; somewhat of a variation as the villains are exploiting Vash's reputation as a ruthless homicidal maniac.
 * In the first chapter of Trigun Maximum, a man who looks exactly like Vash appears, terrorizing a small town. Cue Oh Crap reaction when the real Vash, living in obscurity under an alias comes after Fake Vash.
 * In the Black Cat manga, a Sweeper claims to be The Black Cat to get criminals to surrender without a fight and claim the rewards. He stops only when he discovers that some criminals instead want to show themselves the top gunman instead, and Train saves his life. (In the anime, it's Eve who saves him.)
 * One Piece: A bunch of mooks have decided to cash in on the Straw Hats' name, complete with the appropriate costumes for each character, and get themselves an incredibly powerful crew ripe for entering the New World. Their only problem?
 * Happens at least twice, probably more than that, in Rurouni Kenshin. Kenshin is famous and infamous, as both a ruthless manslayer from the days of the Meji Restoration, and as the unstoppable hero who brought down the government and ensured the success of the Restoration -- after the war ended, he just disappeared, and nobody knew what happened to him. Thus, he's a perfect target for anyone who wants an instant rep as either a killer or a hero.
 * The very first episode has a villain claim to be him while killing people at random in order to ruin the reputation of a certain school of swordsmanship (which he claims to belong to). Fortunately, Kenshin just wandered into town, and sets him straight.
 * Much later, when the Kenshingumi is heading home after saving the country for the third time, they run across an old man in a small village who is claiming to be Kenshin and telling tales of his heroism to everyone willing to listen, convincing them to give him free food and even money. Kenshin's friends are outraged, but Kenshin himself notes that the man is using the goods and money to support a small orphanage, while scaring off local bullies with the threat of unleashing his killing blade, and decides to leave him alone -- he's got no attachment to the name of 'Himura Battosai' anyway, after all. (Of course, inevitably, it doesn't work out that way. An evil swordsman hires on with a local gang, and decides that he wants to test the strength of the legendary manslayer... good thing the real deal was around to provide a demonstration.)
 * One of the Ninku OVAs has a band of circus performers who imitate the heroes to get a cushy job as protectors of a small village. It goes well for them until a band of thieves send word that they're planning to rob the village. The performers try to sneak away, but bump into the thieves just outside the village. They try to bluff the thieves into running away, but they turn out to be skilled martial artists who are eager to fight the real Ninku. Fortunately, the real Ninku step in to bail them out.
 * Happens in Death Note with.
 * The first episode of Fairy Tail has someone pose as Salamander, offering to bring Lucy to the titular guild. He's actually a human trafficker, and is stopped with the help of the real Dragon Salamander, Natsu

Audio Drama

 * The Big Finish Doctor Who audio drama The One Doctor

Film

 * In the 1971 film Support Your Local Gunfighter, a con man (James Garner) enters a frontier mining town and tries to pull a con, having a dimwitted associate (Jack Elam) pretend to be a famous and feared gunfighter, 'Swifty' Morgan. All goes well until the real 'Swifty' Morgan (Chuck Connors) shows up. Hilarity Ensues.
 * In Pirates of the Caribbean: On Stranger Tides, Joshamee Gibbs arrives to London when he hears that Jack Sparrow has a ship and is putting together a crew for a trip. This comes as a surprise to the real Captain Jack Sparrow who has neither a ship nor a crew. He finds out that has been impersonating him.

Literature

 * "The Oracle at Delphi" by Agatha Christie: An American tourist's son is kidnapped, but fortunately the famous problem-solver Parker Pyne is holidaying at the same hotel, and offers his assistance. Even more fortunately, the real Parker Pyne is also holidaying at the same hotel (under an assumed name to avoid people unloading their problems on him), and intervenes before the fake absconds with the ransom.
 * In The Adventures of Huckleberry Finn, the con men pretend to be a recently deceased rich man's brother so they can get his money. Of course, the real guy shows up a few days later.
 * In The Amazing Maurice and His Educated Rodents, the protagonists are the impostors. Then the real Rat Piper shows up...
 * Angel had a tie-in novel with Angel dealing with an Angel imposter running around.

Live Action TV

 * The A-Team, episode "Showdown!"
 * Star Trek: Voyager, episode "Live Fast and Prosper"
 * The Thunderbirds episode "The Impostors".
 * Highlander episode "The Messenger". Someone was impersonating Methos and getting immortals killed telling them to lay down their swords. The real Methos found this amusing.

Video Games

 * Tales of Symphonia has impostors of the main characters trying to con the gullible townspeople. You only have to encounter them once, but you can run into them more later on if you want.
 * Skies of Arcadia featured a Bonus Boss consisting of a bunch of actors impersonating the heroes and using their reputation to rob people. When you eventually fight them, each one have approximately three times the stats of the person they're supposed to impersonate in order to provide a challenging boss fight... Which raises the question of why they simply didn't enter the hero business and made a name for themselves on their own merits.
 * Phoenix Wright: In the third case of Phoenix Wright: Trials and Tribulations, Phoenix's reputation is tarnished when Furio Tigre impersonates him and . Phoenix is understandably annoyed that no one recognizes the hardly-identical stranger as an impostor because he has spiky hair and a defense attorney's badge (made of cardboard).
 * Phantasy Star Zero has a quest in which three unknowns impersonate Kai, Sarisa, and Ogi, commit various indecencies towards the townspeople, and take your own quest ahead of you. Their disguises each have minor cosmetic flaws, but biggest flaw is that they did not impersonate you yourself.
 * Final Fantasy Tactics a 2: After being (mistakenly) accused of an attempt on a noble's life, Vaan and Penelo gain no small amount of notoriety, and one storyline mission has you dealing with a pair of (really bad) impersonators and their lackeys, just before the real deal shows up.
 * In Metal Gear Solid 2, the leader of the terrorists is impersonating Solid Snake, to take advantage of his fame.
 * In Dragon Quest IV, princess Alena and her companions encounter con artists pretending to be them,.
 * Phantom Brave contains a werewolf claiming to be the legendary swordsman Raphael. The second time you meet him, the real Raphael comes to straighten him out.
 * In Suikoden 2 you can run into Hoi (and recruit him) who impersonating the main character to get free meals from innkeepers