The whole question of uniqueness here is rather trivial. As I think Nietzsche said in "On Truth and Lies in a Nonmoral Sense," the more we squint at anything, the more its particularities stand out and its generality or typicality falls away. So absolutely everything is unique in so far as it's an individual thing.

Chesterton's point is that only Christianity said God became a man. That's an empirical statement that rises or falls with the evidence from history. The problem is that there are degrees of divinity and incarnation, as in the case of Greek demigods. There is indeed a mytheme of dying and rising savior demigods, and however we might emphasize Jesus' uniqueness, we could just as easily emphasize his typicality.