Steed had several doppelgangers during The Avengers.
1. The first one appeared in Man with Two Shadows in series two.
He has trained five years to become John Steed because Steed is such a complex character with "many talents." But the original Steed kills him when his double tries to murder the agent. But afterward, Mrs. Gale is not sure if Steed is the real one and intends to execute him.
2. In series 4, episode Two's a Crowd.
Mrs. Peel is afraid that Steed has been killed and replaced with a double by PSEV and Brodny, but John Steed finessed them because he was the "perfect double" Webster himself.
Trivia: Emma Peel believes that the real Steed has been killed and is about to shot the "false" one. She should have better checked his Adam's apple properly! The tie pin which Steed is wearing was a gift from Mrs. Peel for either Christmas, Birthday, or a "special occasion" - I see!
3. The third double was in series five episode Never, Never Say Die.
It is not made of flesh and blood but is a robot. Dr. Stone created Steed's robot double, but it was rejected because it was not perfect. Even Mrs. Peel criticizes the low quality of the duplicate, and she should know.
Trivia: According to the original script Steed's and Emma's duplicates were supposed to fight against real Steed and Mrs. Peel, but Steed and Emma Peel find them in a storage section at the end of the episode.
4. Some odd form of double can be seen in series 4 in Who's Who???
This time not only is Steed the victim, but Mrs. Peel too, when Basil and Lola, two foreign agents, swap the bodies with Steed and Emma, using Dr. Krelmar's machine that transfers the mind, the soul and the entire psyche of one person to another.
5. The next doubles appear during the Tara King series in They Keep Killing Steed.
Eastern agents developed a formula that allows "instant plastic surgery," which transforms the face of someone into the face of another person. The attempt is made to infiltrate a peace conference where false Steed is supposed to set a bomb and to blow up all the members of the meeting. But Steed can escape the people who took him captive and prevents the disaster. There're several doubles in They Keep Killing Steed, they all look like Steed, but the voices do not match. All get killed or die due to false medication.
6. The last time someone tried to copy a Steed was in TNA Faces.
Hostile agents are looking for people who look similar to people in important and high positions of the British government. By using plastic surgery, they transform these people and switch them with the original ones. Steed becomes suspicious when his oldest friend, Mark Clifford, a high official of the government, suddenly dies. Steed and Clifford's wife realize that the corpse is not the real Clifford. But there also exists a double for Steed, who tries to kill the agent and replace him afterward. But Steed is lucky, and the golden Hunter watch from his deceased friend saves Steed's life when the bullet hits the clock and not Steed's heart.
Several Steed dummies are located on the exercise terrain in TNA Target! Gambit shot one Steed!
"Irreplaceable" Steed from Faces