Our first clue about the dualism is in the Koran, which is actually two books, the Koran of Mecca (early) and the Koran of Medina (later). The insight into the logic of the Koran comes from the large numbers of contradictions in it. On the surface, Islam resolves these contradictions by resorting to “abrogation”. This means that the verse written later supersedes the earlier verse. But in fact, since the Koran is considered by Muslims to be the perfect word of Allah, both verses are sacred and true. The later verse is “better,” but the earlier verse cannot be wrong since Allah is perfect. This is the foundation of dualism. Both verses are “right.” Both sides of the contradiction are true in dualistic logic. The circumstances govern which verse is used.
That can be pretty convenient, if you ask me.