X-Men and the Micronauts #4 (1984)

Doppelganger!
The Entity has taken over Xaviers body and proceeds to take over the New Mutants. In the Microverse, Karza (in Kittys body) attacks the X-Men, whereas Kitty (posing as Karza) leads Karzas battlefleet to attack the entitys world. Kitty orders the bombardment stopped, teleports down and confronts Karza. The X-Men, of course, misunderstand the scene, until Xavier, who has now taken over the Entitys failing body, appeals to them and explains the situation. On Earth, the Entity uses Cerebro to amplify his powers and starts destroying the Microverse, starting with Karzas fleet and the world the heroes were just on. They flee in the Micronauts Bioship and, with the help of Fireflyte, return to Earth and Xaviers School, where they are attacked by the New Mutants. The heroes keep the possessed teenagers, busy while Xavier secretly enters the Entitys mind, intent on purging him from his body. He plays a game of psychic chicken with the Entity by convincing him that he is willing to cause a stroke in his own body, to beat the Entity. The Entity flees back into his own body that has died and ceases to exist. The badly damaged Bioship returns the X-Men to their natural height before it dies and Fireflyte returns the Micronauts to the Microverse.
Release Date: 1984