Perry Zirkel has written an article about his study of self-plagiarism. This is an excerpt:
Doing a review of the literature for an article in my area of specialization in education. I came across two recent journal articles that were surprisingly similar at their core. One was co-written by Professors Alpha and Beta; the other by Professors Gamma and Beta.
The core section, which amounted to approximately a quarter of the text in Gamma and Beta's article and almost half the text of the Alpha and Beta article, was identical, word for word. Yet neither article cited the other. The articles, both appearing during the current year, were in different refereed journals.
Examining this situation crystallized two issues in my mind -- one immediate and the other more general. For the immediate issue, I explored some sources in the social science literature that identified the problem as possible "self-plagiarism," which on first impression sounds like an oxymoron. As a matter of intellectual ownership, how could one plagiarize one's own ideas?
BTW, Margaret Munro has recently also written a piece about self-plagiarism by a researcher at Queen's University in Kingston, Ontario.