Tales of the Legion of Super-Heroes in"The Super-Villains of All Ages" and "The Civil War of the Legion!"
*Posing as a Legion applicant, the villainous Alaktor gains entry to the Legion Clubhouse, and secretly takes pictures that allow him to circumvent their security system and steal a Time Bubble. Traveling to the past, he gathers Emperor Nero, John Dillinger, and Adolph Hitler, and constructs a device that transfers their personalities into the bodies of Mon-El, Ultra Boy, and Superboy respectively.
Together, the evil trio gathers the mighty scientific machines of the Lost World for Alaktor, nearly destroying their former teammates in the process, until Saturn Girl uses her mental powers to turn the three possessed heroes against each other. This allows the Legion to apprehend Alaktor and force him to return the evil personalities to their proper hosts. *Source: The Official Legion of Super-Heroes Index #2 - Published by ICG.
Superboy in "My Son, the Boy of Steel".
Smallville citizen Amos Carter, believes that his son Fred is actually Superboy.