"And before you were tellin' me you didn't know what I was talking about with the déjà vu thing." Sans doesn't sound surprised, obviously. He's also pretty sure that, the way she's referring to the mysterious second Sans, he either already agreed to this in another timeline or is going to be intentionally leading himself into here later for a past him to be this current him.

So, Sans hates time loops. But anyway.

"If it's going to get us out of this whole situation faster, I'll give it a shot."

Especially since he was basically already wandering around pretending to be one of the other skeletons to gather info until this whole thing happened.