How is she so identical to the duke, like she straight up another clone of him

I think this may be a descendant thing, in these type of story’s the nobles tend to have a primary god they are descendant of. Even if Ariel is isn’t the one they are a descendant from she may also be a descendant herself of a more ancient god.

So he literally raised his ancestor

In a way , I guess cause they see Ariel as a goddess it’s less like a great grandma and more like the founder of their family type thing