Passage 9.31.8
ἔχει δὲ καὶ ἕτερος ἐς αὐτὸν λόγος, ἧσσον μὲν τοῦ προτέρου γνώριμος, λεγόμενος δὲ καὶ οὗτος, ἀδελφὴν γενέσθαι Ναρκίσσῳ δίδυμον, τά τε ἄλλα ἐς ἅπαν ὅμοιον τὸ εἶδος καὶ ἀμφοτέροις ὡσαύτως κόμην εἶναι καὶ ἐσθῆτα ἐοικυῖαν αὐτοὺς ἐνδύεσθαι καὶ δὴ καὶ ἐπὶ θήραν ἰέναι μετὰ ἀλλήλων· Νάρκισσον δὲ ἐρασθῆναι τῆς ἀδελφῆς, καὶ ὡς ἀπέθανεν ἡ παῖς, φοιτῶντα ἐπὶ τὴν πηγὴν συνιέναι μὲν ὅτι τὴν ἑαυτοῦ σκιὰν ἑώρα, εἶναι δέ οἱ καὶ συνιέντι ῥᾳστώνην τοῦ ἔρωτος ἅτε οὐχ ἑαυτοῦ σκιὰν δοξάζοντι ἀλλὰ εἰκόνα ὁρᾶν τῆς ἀδελφῆς.
There is also a different story about him, less widely known than the previous one but still told, that Narcissus had a twin sister who resembled him exactly in appearance and who wore her hair and clothing in the same way as he did. Indeed, they used to hunt together. Narcissus came to love his sister, and when the girl died, he would visit the spring, aware that he was seeing his own reflection. Yet despite this knowledge, he found relief for his passion by imagining that what he saw was not his own reflection but the image of his sister.