I’ve tested that example on my local dev site (running the latest Discourse code with the Chrome browser.) I’m finding that the second footnote is positioned either at the bottom of the page, or off the page.
If I try inserting a unique footnote between the two duplicate footnotes, the second of the duplicate footnotes will display the content of the unique footnote if it is clicked after having clicked the unique footnote. Example markup for that issue:
We are our choices. [^sartre]
A lot of interesting words
Second footnote test [^second]
He really said it. [^sartre]
[^sartre]: We are our choices.
Sartre, some time
some day
in the past
[^second]: this is the content of the second footnote