[Ah, the question catches Asriel off-guard. Of course Chara wouldn't know who Frisk was. But he shakes his head and tries to stand straight and firm, because he knows by now that something is wrong, he just has no idea what and why.]

That's Frisk's sweater - it's like you're a copy of them! But - but you're not supposed to be, if you're Chara then you're supposed to be Chara!

[He's mixed you both up once before, out of desperation and loneliness, but he knows by now not to do that again.]