I was going through the archives and saw this was never answered. All I can say is there is no easy answer. I think you just have to write what is in your mind without worrying about if you will be plagiarizing. The young woman from Harvard that said her plagiarism could be explained by subconciously not knowing she was doing it is quetionable. I recently heard that John Legend had to pay some 1970s group for borrowing from one of their songs which he claimed he never heard. Who knows wht the story is. If you worry about it you will never be able to write anything. Just document and date your writing and do the best you can.