Although originally a question, I turned this into an answer because I think it is more useful here.
Zoroaster, the prophet said there was One G-d, Ahura Mazdah—in Greek, the Wise Lord. Zoroastrianism felt that there was only one god. Yet some Parsis feel that the Wise Lord's twin brother Ahriman, who is evil, exists. They were both created simultaneously.
At the beginning of creation, there was only infinite time but the Wise Lord set a trap by creating the time of the long dominion within infinite time. The human timeline is encased within the time of the long dominion. When this ends the Wise Lord will defeat his twin and light will burn away the darkness. They say the Wise Lord had no choice in the matter. Thus, they answered that eventually, the Wise Lord will overwhelm evil, and all souls will be as one.
Thus, it seems that Zoroastrianism is closer to dualism than monotheism, in my view. There was a notion of a dual deity, however, many Zoroastrians insist that they are really monotheists.