We think of a poem (or novel or play) as something unreal that imitates something real.
But nothing could be further from the case. A novel is real, it exists. What it represents is a fiction, something that does not exist in real life. Therefore it is useless to say a novel copies life.
Or a poem, it is an utterance that is real in and of itself, but that other utterance it seems to be copying never happened. It is an impossible utterance, or one whose only possible framework is the poem itself. Can we think of Keats copying copying another utterance of some other imaginary subject addressing himself to "Autumn." Does not that introduce an extra step that is wholly extraneous? Where is the model that is being copied? In Keats's head? But it cannot be there unless he first invents it, and what we call inventing it is the same act as writing the poem.
I am not being facetious about this at all. I firmly believe that this analysis is correct, even intuitively correct. It is not even a paradox.