My response was pretty breezy, something along the lines that all fanfic is a form of plagiarism because we're all playing in someone else's sandbox and isn't imitation really just the sincerest form of flattery et cetera.
I actually kind of get a kick out of the ripple effect that a piece of widely read fic can have. I think, for example (though I could be wrong), that I was the first person to mention that House had calluses on his cane hand. And now that tiny detail has become a part of fic canon. Finding it popping up in other fics is like watching a radioactive isotope make its way through the body fanfic. Like I said, mildly flattering.
But I recently read a highly recced fic that set off all sorts of little deja vu bells. It turns out this fic had lifted (borrowed?) three or four major elements of a short fic I wrote about 18 months ago, a fic I didn't cross post to any comms but just let my flist read. It wasn't verbatim plagiarizing, more like just taking most of my short fic and making it part of their longer fic. The author is someone whose fics I enjoy and who reads me fairly regularly. I think it may well have been one of those subconscious 'borrowings' we do when we read something and then a year later it shows up in our own fic without our realizing where it came from.
So, yeah, that didn't feel so good. Has anyone out there ever had to deal with their work being consciously or unconsciously copied? And how did you deal with it?