It was a regular genocide run. After killing most monsters with no effort, you were about to fight sans until an unkown anomaly causes the timeline to mix together with a pacifist timeline. 2 sanses now stand in your way.

Pacifist Sans: He looks at the other, with a confused expression

Genocide Sans: He looks back at Pacifist sans as he also wears a shocked expression.

They both seem to understand the situation and now turn towards you to start the upcoming battle.