I'm not sure I understand how anyone can plagiarize themselves either. Isn't that just called "recycling your own material"? For example, several parts of David Foster Wallace's speech/book This Is Water are pretty much verbatim from Infinite Jest. Or what about authors who are pushing ideologies and repeat themselves a lot? For example, there's a lot of overlap between Ayn Rand's novels Atlas Shrugged and The Fountainhead. Isn't this just what writers do?