Multiple calls to the same footnote

[Omikhleia]

Just some ideas following a personal discussion.

(In Djot at least) nothing forbids a writer to call the same footnote reference multiple times.

There are some cases where this could be useful (ex. a list of items, where several call for the same details in a common footnote). One could argue that's it's only an "acceptable" solution if the calls are on the same page (or not to far away...), but that should be an author decision, not a technical limitation.

Currently, we do not reject multiple calls to the same note, but:

• A warning is issued (Duplicate label '...': this is possibly an error)
• Two footnotes are produced (with same content... and same cross-reference label, so only the latest will be honored if the footnote is later referred by its identifier).

It would be possible to support this is use case:

• On first occurrence, spit out a complete footnote indeed.
• On later occurrences, only spit out a footnote reference call (to the first call, via cross-refs)

One problem however is that the footnote reference call are not the same between SILE's regular footnote package (footnotemark) and the resilient footnote package (style-aware footnote:reference) -- They are not honoring the same "public" API here :)