Multiple References to the Same Footnote Results in Duplicate Footnotes

### Expected
When referencing the same footnote more than once, both references should point to the same footnote.

### Observed
When referencing the same footnote more than once, a duplicate footnote is created. Reported here: https://forums.simplenote.com/forums/topic/multiple-references-to-the-same-footnote-results-in-duplicate-footnotes/?view=all#post-2826

### Reproduced
I can reproduce this with the following steps:

1. Open a note
2. Add text with Markdown formatting for footnotes (e.g `[^1]`, `[^2]`)
3. Publish note
4. View the published note

***Screen recording***

https://github.com/Automattic/simplenote-electron/assets/35628116/070bbffb-d448-4a12-9387-945fae05aa74

### Where did you see the bug
- Device: MacBook Pro 2021
- OS: macOS
- OS version: 14.5
- Browser (if applicable): Chrome
- Browser version (if applicable): 126.0.6478.127
- Simplenote app version: 2.21.0

Reproduce the issue in the Simplenote app using one footnote referenced more than once, then inspect the published-note rendering path and its Markdown footnote handling. Done means repeated references point to one footnote instead of creating duplicate footnotes, while the documented reproduction steps continue to work.

Written by the indexing model from the issue text.