You know, at one point here on one of my hubs - I'd typed something out, and it came up claiming that it was duplicate. I couldn't believe it. I was super shocked. Sometimes I'd copy/pasted a paragraph of something somewhere, etc - and of course I knew that that was something on another site.
Bottom line here is that we are dealing with machines before a moderator has seen anything. There are plenty of words in the English language, and we can always word something a bit differently and leave it saying almost exactly, or exactly the same thing.