I'm sure that it happens. I wonder about unique ideas though, You might have those, don't get me wrong.
Me? Sometimes I do think that I've thought up something unique, then later on I go, "naw, that idea was obvious, you just think you're smart, Todd."
It would be VERY hard to prove that an idea was unique. I'd imagine that it might be impossible. Now, of course if you're idea leads to a product of some kind, or something; then that could be patented. . . .But hey, you could try to use the original publication date as proof of an idea's origin?