Redundant οὐ appears after the negative words πλήν, χωρίς, ἐκτός, ἄνευ except, without, and after πρίν (and μᾶλλον ἤ usually) preceded by a negative, which may be involved in a question.
νῦν δὲ φαίνεται (ἡ ναῦς) . . . πλέουσα πανταχόσε πλὴν οὐκ εἰς Ἀθήνᾱς
but now it seems that the ship is sailing everywhere except to Athens 56.23, πρὶν δ᾽ οὐδὲν ὀρθῶς εἰδέναι, τί σοι πλέον λῡπουμένῃ γένοιτ᾽ ἄν; before thou knowest the facts, what can sorrow avail thee? Hel. 322, εὖ δ᾽ ἴστε ὅτι οὐ περὶ τῶν ἐμῶν ἰδίων μᾶλλον τῑμωρήσεσθε Πολυκλέᾱ ἢ οὐχ ὑπέρ ῡ̔μῶν αὐτῶν
but be assured that you will punish Polycles rather for your own good than for my private interests 50.66. Cp. “j'irai vous voir avant que vous ne preniez aucune résolution,” “le bon Dieu est cent fois meilleur qu'on ne le dit.”