Superboy (1994) 63-A

"Hyper-Tension!, Part 4 of 5: For Every World...A Doomsday!"

Black Zero insists that he's not a clone and that he only wants to protect to Superboy clones in every world.
Knockout along with the Challengers of the Unknown crash the party and a battle ensues.
When Westfield disintegrates after being thrown atop the Hyperium, Metron reveals that any misstep in hyperspace can destroy all duplicates forever. Wishing to test this theory Black Zero grabs Knockout, and the heroes have no chose but to surrender. Black Zero locks the heroes in the Doomsday chamber. While in the chamber Superboy discovers that the Challengers are actually the originals from his world.
Superboy uses his TK to open the door, but to prevent the Doomsdays from escaping Knockout sacrifices herself to hold them off.