As someone who's been seriously accused of plagiarism (not in fandom, obviously - I'd have to write fic or something for that!), as in almost kicked out of uni serious, I did (well, me and a few of my friends) quite a lot of research into it and I'd say that as long as something is credited, it's not plagiarised.

Would also like to add that I was innocent, both of us were, it was just completely freaky how similar our lab reports were.