οὐκοῦν εἰς μὲν ταῦθ᾽, ὥσπερ εἴπομεν, ἀλλήλοις τε ὁμολογοῦσι καὶ αὐτοὶ ἑαυτοῖς ἰδίᾳ, καὶ δημοσίᾳ αἱ πόλεις πρὸς ἀλλήλας οὐκ ἀμφισβητοῦσιν αἱ μὲν ταῦθ᾽ αἱ δ᾽ ἄλλα φάσκουσαι;
the two negatives οὐκοῦν οὐκ become positive?
these interrogative particles require a positive answer, like nonne, but here, due to the negative meaning of ἀμφισβητοῦσιν the positive answer would be impossible.