Repeated footnotes

I’m using footnotes as references to quote scientific literature. If I am trying to repeat a citation it creates a brand new citation. For example, the reference associated with 1 & 2. If since last referencing them I’ve gotten up to reference 4 and 5. When I want to reference 1 & 2 again it generates a new footnote, 6 & 7 which are a replica of 1 & 2. Does that make sense? How do I simply show the superscript of 1 &2 without creating a new footnote?

https://wordpress.org/plugins/footnotes/