You formulated it as though you were bringing up something new: "you mean the same X who Y" is for introducing something new into the conversation in relation to X, with X here being Jill Stein. If you had just used David Duke as X and "who lead the KKK" as Y, it wouldn't have been an absurd contribution.

Though it would still be a silly one, since people know who David Duke is, it's not some obscure fact. He's the single most recognizable name in connection with the KKK, perhaps along with the long-dead D.W. Griffith (but probably not).