Stories are told of musical prodigies who could, on hearing a piece of music performed, could write down a note-accurate score of the piece. Ansel Adams famously referred to the print as a performance of the "score" inhering in the negative.
Happily, we needn't be a photographic Mozart to write down the score of a print. We need be merely a competent technician with fair observational skills. In general, a moderately skilled photographer can, with an existing photograph for reference, reproduce it. With landscapes there are, of course, issues with weather and growing trees and so forth. With a model you probably have to hire someone else, and their makeup will never be quite the same and so forth. Although carbon copies are possible, what I am really interested in is photographs that function in the same way as the original, the light falls the same way, the mood evoked is the same, and so forth. One places a light here, a light there, tries such-and-such a focal length -- oh bother, I need to be a little longer -- and so on. The amount of hacking around depends on the reference image and the photographer, but it can be done.
Let us not forget post processing, of course. The actual printing of the negative, the monkeying about in a digital photo editor. Let us nod to it, and wave it past as all, essentially, technical details which can also be seen in the print and emulated.
Shooting a fake, albeit minor, Avedon isn't even particularly hard. Painting a fake Sargent that would pass even casual muster is a substantially more challenging proposition.
Essentially, most photographic ideas and looks can be reduced to a formula, which is then more or less straightforward to follow.
The professional photographer, being first and foremost in business, is selling a product. The essence of a product lies in reproducibility. The ability to create photographs per a formula is, then, more or less essential to the business of photography. If the pictures from each wedding came out quite differently, the wedding photographer would shortly be out of business (or at any rate, would suffer). There are many reasons people rarely pay to have their weddings painted, but surely one of them is that you haven't a clear idea what it is you're likely to get in the end.
The best professionals will, of course, create their own formulae, new formulae. These will become products in their portfolio, a new "look" the clients can purchase. There is originality here, albeit usually evolutionary, but the professional is forced by business reality to reduce it to formula.
It follows that original work in photography lies mostly in the domain of the amateur and the artist. Professionals and amateurs can, of course, be artists. Earning one's bread with the camera does not exclude one from the exclusive club of original thinkers. Rarely, though, will the professional be able to express those original thoughts while simultaneously earning that bread.