That's the subject of this interesting slide show at Slate.Com entitled,
Can Photographers Be Plagiarists? (warning: brief nudity....
that ought to increase the click-count).
I've seen enough lure coursing photos by other photographers.. (both full time photographers, and part time photog... oh wait.. with one exception, I'm the only full time photographer shooting sighthound performance portraits. Freakin' scabs!)... to know that they can sometimes look alike, and very occasionally, I have trouble knowing if a shot is mine or one of the wannabes out there. (But only very occasionally..LOL; Y'know, sighthounds- and other canines- can only be in just so many poses or environments.)
When we were at the Gila National Wilderness a couple of weeks ago, walking the Whitewater Creek Gorge, I walked past an elderly gentleman who had set up his tripod, with Nikon D200 aboard. I didn't look at what he was shooting, because I was going for a more (hopefully) unique shot from the bottom of the gorge, at creek-level. When I got back up to the trail, I glanced across to see what he had been shooting. I liked it. Here is the shot:
Is it plagiarism? Judging from the amount of trampled earth, and lack vegetation between the trail and the subject, I'd say no. It's just appreciation of a nice image by probably hundreds- if not thousands- of photographers- amateurs, professionals, semi-professionals, and just plain tourists.