Si m'ait Dieus
If you go back to the Gathas, there was no dualism. Zarathustra taught that God created two spirits, one of whom rebelled. Later believers confused the Good Spirit who remained faithful with God and ended up with the idea of two equal spirits.
He also acknowledged the existence of subordinate divinities: " We worship Ahura Mazda and his Spirits; we worship the entire creation of God, bother the spiritual and the physical." (Yasna 35) So, not the same sort of monotheism as Islam, or even Christianity.
One passage that Aupmanyav might find interesting is Yasna 37.1 where we are told that God created "the cow and the righteous man" -- note the order of precedence.
Reading the Gathas is terribly difficult, because there's no literature of the same period to clarify the meanings of words. We can compare Avestan to Vedic Sanskrit, but for Cyrus, it would have been like a modern trying to read Old English without a grammar or dictionary: God ana wat hwa þære wæstowe wealdan mote! (i.e. God alone knows how things will turn out.)
How can you talk about Gathic Zoroastrianism then reference Yasna 37? There are only 17 hymns. Also, I'd like the Gathic quote for this:
"...Zarathustra taught that God created two spirits, one of whom rebelled..."