I'm Fighting Myself? Is This for Real?

One day, the sky began to shatter.

Different worlds began to converge, and another you descended.

He might be the archbishop of a Holy Church in a fantasy world, a duke of demons in the abyss, or a Heavenly Machine warrior from a science fiction world. But no matter where he comes from, only one of you can exist in this world.

Fight! Fight yourself. The defeated one's existence is the world's way of acknowledging you, while victory is the only existence recognized by this cruel world.

(The overlapping of worlds causes souls to overlap. Two identical souls cannot both be recognized by the world. If one is not eliminated immediately, the world will erase them both. Under the world's gaze, identical souls are fated to fight to the death.)