Can you self-plagiarize within the same document? The title is the question: can you plagiarize yourself within the same document? For example within a book of thousand pages, do you have to reference ideas from page 50 when you use them on page 950?
The short answer is: No, it is not (self-)plagiarism because it is the same book.

The longer answer is that a reader may want to know more about the subject when reading only the chapter on page 950. It would be highly beneficial for him/her to have a reference to the earlier section on page 50 where more information or more detail about the subject is given.

Finally, you should consider whether it is really necessary to literally repeat yourself within the same book (hint: probably not). This could be an indication of a sub-optimal structure of the book, or maybe providing a short summary and referring to the earlier section for more details is enough.