How to handle repeated references in footnotes?
Generally speaking, you should use whichever style sheet your publication venue asks you to use. Different styles have different ways to handle this.

As an example, the following are two common ways to deal with repeated references in footnotes:


Use "op. cit." to refer to the prior reference by the same author.
Use "supra X", "supra note X", to refer to a prior footnote, where X is the number of the footnote. This is relatively common in law. You can also provide different page numbers by using "supra note X, at Y" where Y is the page number.