Imagine a big scientific lab that puts out a lot of publications, some of which are spin-offs of the main agenda of the lab. I've been worried that my publications were duplicating themselves a little too much, but I've decided that I'm fine. There will be a little redundancy and overlap because the corpus I'm working on suggests additional ideas, because books will repeat arguments made in articles, etc... I might write a second article making the same theoretical point, but using other texts, for instance. I feel bad sometimes when I do this, but looking back on previous cases I don't think I'm all that reduplicative.
A single scholar is a like a big laboratory in this respect. This is also why it's easier to publish a lot more if you are already publishing a lot. What is more difficult is going from zero to one, going from publishing nothing (or a very small amount) to publishing a moderate amount. If you are actively engaged in research, you will get extra ideas you won't even be able to use, because you will notice interesting things in other texts you are reading.