I think great art develops through a sort of evolutionary process. Everyone is influenced by the writer's they admire and so the good ideas get replicated. But good ideas by themselves aren't enough to attract an audience, so authors have to creatively combine and mutate those ideas. Harold Bloom called this the "anxiety of influence". The ideas they copy might be stolen, but the ways they combine those ideas might not be.
Questlove from The Roots recently published a really good book on creativity (Creative Quest) where he goes into exactly those points in detail. He doesn't see himself as a terribly creative drummer, but he knows so much about other drummers and can replicate and merge their styles so well that it seems extremely creative to the listener, because the listener doesn't know these different styles. Then he goes on to say that this happens with all artists, especially now where Twitter etc. keep on exposing you to all kinds of ideas and sets from other artists. The artist moves from creator to curator.
I believe it's very similar in some corners of literature, and it's not a new phenomenon. Think of Umberto Eco's novels, he was extremely widely read including some obscure niches (medieval philosophy) which influenced plots and motifs etc. in his novels (think Baudolino, or imagine The Name Of The Rose without Borges' influence!)
This gets into what Picasso meant by "Good artists copy, great artists steal".
I take that as meaning that a great artist makes the material wholly their own through transformation, while a good artist merely alters the original which remains identifiable.
