One word seems to me somehow very strange with its dual number. It is in a passage from Plato's Philebus:
φωνὴ μὲν ἡμῖν ἐστί που μία διὰ τοῦ στόματος ἰοῦσα, καὶ ἄπειρος αὖ πλήθει, πάντων τε καὶ ἑκάστου.
with a translation:
Sound, which passes out through the mouth of each and all of us, is one, and yet again it is infinite in number.
The word πλήθει is, as far as I've checked, a dual word of πλῆθος, meaning "a great number". I would think that the word shall be used as either singular or plural. Is there a reason that this word is especially dual?