Ensure refs are unique on export

[alexfikl]

In general, we do not enforce that ref is unique in a library. However, when exporting to various formats, we should ensure that all the documents have a unique ref.

This should apply to

• [ ] BibTeX
• [ ] Typst / Hayagriva

xref https://github.com/papis/papis/discussions/935

[tuurep]

Hmm this is somewhat related to #988, #1038

Should we make export error out on:

• having many docs with same ref key
• appending duplicates

?

[alexfikl]

Should we make export error out on:

I'm not sure what a good behavior would be here, since you can always delete/rename them after the fact if you need them to be unique. From what I can tell:

• You can export things to use them in a TeX documents, which will complain about duplicates.
• You can export things just to have a backup, in which case duplicate refs don't matter that much.

So maybe a warning would be appropriate?

This also raises the issue of what to do on --append, since ensuring no duplicates means we need to read the existing file in, check it, possibly fix it, append the new documents, and write it back out.

[jghauser]

Could we leverage papis doctor functionality for this? Maybe this could be enabled by setting auto-doctor to true or with a --auto-doctor flag (though in this case it would just check the refs?).

Somewhat relatedly, I think we might eventually want to have auto-doctor default to true