Self-plagiarism, discussed here in a post from language log, is something that I have to avoid, since I am the type of scholar who returns to the same ideas and authors repeatedly. Since people only read all of what I write when I am up for promotion (something that will never happen again) I think it's fine to duplicate ideas in different contexts, as long as I cite myself amply. I have eight work of JM in my current bibliography for my project. Looking at it now I see I need to include at least two or three additional books and articles. Instead of seeing that as a sign of egotism, it could be a sign of a certain honesty in revealing that I draw from my own previous ideas and obsessions.
I often feel there is more duplication than there really is. For example, I could think that three chapters of my book Twilight are ways of saying the same thing in different ways: "The Avant-Garde and Its Discontents," "Three Apologies for Poetry," and "Poetry, Politics, and Power." But really, they represent distinct analyses. The same goes for my two essays on Valente and his precursors: one looks at Celan, the other at Beckett. The structure of the argument is very similar, but I think writing both was justifiable.
Whenever you find someone who has published a lot, you will find some duplication of intellectual frameworks and ideas, modes of analysis. Often I stop reading a book when I feel I've gotten all that it offers, even at the expense of some additional information or analysis. I wouldn't be surprised or unhappy if others treated my work like that.
I could go back and publish a book out of my remnants, articles that never became book chapters, but I don't think it's worth it. I like the fact I have articles that never found a home in a book. I do think there is enough for a book there, though. I just went back and counted 17 articles. The problem, paradoxically, is that they aren't either coherent or miscellaneous enough. Any way of grouping them would reveal too many connections, but there would be too many things that did not fit.