Once More, with Feeling
Sometimes I get too self-aware and think I repeat myself too much. Most of my writings tend to circle around the same half dozen topics, revisited rehashed rediscussed from one end to another. Sometimes I change minds and contradict my own self in the past. Sometimes I gain a new resolve. It's part of the beauty of being human, I think, that we grow and step into a new self in this circular path.
Mulling over the same thing can be unhealthy, but writing (and speaking, if it suits you more) is a method for us to process whatever is percolating in our brains. We shan't be so afraid of repeating ourselves. In fact, I'm partial to the notion of revisiting the central themes of our philosophy more often.
Perhaps it was academia ruining it for us. School drilled in our mind the sin of self-plagiarizing, but outside the labyrinthine world of citations and attributions, what difference does it make from continually publishing working drafts? Our daily life is not plagued by publish or perish (or at least, I sure hope so). Your writing does not have to contain new thoughts. If your findings after several years are nothing more than "this still holds true for me", that's a valuable insight in regards to our life as a whole.
Many authors do in fact write more than one book dealing with a certain topic. Sometimes they are merely quick cash grabs designed to follow after a particularly good reception, sometimes they are the kind of beast we just described. Deeper take, shallower scope, entirely different viewpoint. To turn the topic around, a marble in our hands, to polish it, test it, stretch it, ultimately finding a little bit of truth in it.
Perhaps by doing so, we can inch closer to the form we seek in our mind's eye.