How much of your writing belongs to you?

Hey everyone, just a question that’s been on my mind. Sometimes I’ll introduce a character or plot element into my writing that feels at place in the piece but alien to the way I write. I’ll look back at a sentence or phrase, and feel like someone else ghost wrote it, or that it spun organically out of the material. How much of your writing feels like it came from somewhere else? What role does that feeling play in your writing process?

David