Label `<B>` occurs multiple times in the document when including #outline

Questions
1

I have numerous JSON files that I’m loading and processing with a function. Each JSON file gets its own heading and label, and some files will refer to others. My minimum reproducible example has two files.

a.json

{
    "title": "A",
    "refId": "B"
}

b.json

{
    "title": "B"
}

Then, my Typst file (pared down from the full project):

#set page(numbering: "1")

//#outline()

#let get-ref(ref-id) = [
  #context {
    let loc = locate(label(ref-id))
    let pn = loc.page-numbering()
    let nums = counter(page).at(loc).at(0)

    let a-text = numbering(pn, nums)
    link(loc, strong(ref-id) + [_(page _] + emph(a-text) + emph[)])
  }
]

#let show-item(..args) = [
  #let pos = args.pos();
  #let named = args.named();

  #let name = named.at("title")
  === #name #label(name)
  #let refId = named.at("refId", default: none)
  #if refId != none [
    See also  #get-ref(str(refId))
  ]
]

#show-item(..json("a.json")) // has "refId": "B"
#show-item(..json("b.json")) // has no refId

If I run it like this, it works. If I uncomment this line:

#outline()

Then I get this error:

 error: label `<B>` occurs multiple times in the document
  ┌─ \\path\to\main.typ:7:14
  │
7 │     let loc = locate(label(ref-id))
  │               ^^^^^^^^^^^^^^^^^^^^^

This certainly seems like a bug, but maybe I’m misunderstanding how labels should work?

2

This places the label inside the heading’s body (because you’re using the label function, not label syntax), and that should explain why you get duplicate labels: the outline also reproduces all heading titles (which now include labelled words).

It’s a bit awkward, but this is a way to attach the label to the heading while still using typst markup for the heading:

// in typst code mode, create heading with label
[#[=== #name]#label(name)]

I think that should hopefully explain what you see and give a way to resolve it.

3

Uuuuh, what? That doesn’t feel like an expected or wanted behavior. There is little to no difference for using either form for different elements, so this is an outlier. A change request probably, so I guess a feature request.

An oversight? If anything, I’d think that <label> is also “a part of heading’s body.” Though from “attachment” perspective it’s definitely a separate thing.

If I do #panic([#label("label")].func()), then it acts exactly like <label> and results in empty sequence.

4

It’s well described here Label Type – Typst Documentation so it’s at least well known.

5

Hmm, so it’s also kinda like the figure[] + label("a") issue in code mode. Rather an annoying edge case for scripting and automation.

Well, at least = #label("a") throws a warning. That’s something too. It’s also surprising to find such a niche example explicitly present in the docs.