I posted a short story here recently and then another Lit-Nutter Pm'ed me to let me know that they were actually writing a book on the same theme. Of course there were differences they said but they did not want me to think they had stolen my idea if I ever see it in print.
This is a courteous thing to do of course.
But what happens if I read a great story here and decided 'Hmm I could write a book on that plot' and so PM the poster to let them know I already had a similar idea and I don't want them to think I'm stealing their ideas?
Then again, how many stories are there about love? I think I've missed the point on plagiarism completely so I'll stop now