I don't usually re-purchase books I own in paper as ebooks, though I do try to acquire them in other ways, because my current house has very limited shelf space, most of my paper books are in storage, and I like to re-read. But I feel that I've already paid for one license to read the books I own in paper. (Especially since many of my paper books-- including those I like to re-read-- aren't available as commercial ebooks anyway.)
I would consider buying an ebook of a book I already owned in paper if there was additional content, e.g. author interviews, hotlinks between unusual terms in the book and a glossary or map, new background content provided by the author, etc.