How is the witches’ prophecy “Fair is foul, foul is fair” repeated throughout Act I? Or how is the idea of “Fair is foul, foul is fair” emphasized?

I'm not exactly sure what you are asking here. The witch's chants have to do with the concept of equivocation, seemingly contradictory meanings. The witches are addressing multiple subjects here. They are addressing Macbeth (good and bad) as well as the weather.