Q. For citations, a quoted sentence A is marked with a footnote of no. 8 with a particular author, book, and page number. Sentence B in another paragraph is also quoted from the same author, book, and page number. Can we label it with footnote no. 8 or do we have to cite it with another number?
A. There is a system by which superscript (or parenthetical) numbers refer to a numbered list of references, and under that system you could use the number 8 for all citations of a given author and book (please see CMOS 14.3). This is not Chicago style, however. In Chicago style every note number is unique, so you would go to note 9 in the case of a subsequent reference to the same book (if there are no other notes in between).