Duplicate class for unnumbered headers in HTML

[marrus-sh]

When I type:

echo '[Header 1 ("section",["unnumbered"],[]) [Str "1"]]' | pandoc -f native -t html

I get:

 <h1 class="unnumbered" id="section" class="unnumbered">1</h1>

As you can see, the class attribute is listed twice, which is bad.

pandoc 2.9.1.1
Compiled with pandoc-types 1.20, texmath 0.12, skylighting 0.8.3

(macOS 10.15.2; installed via homebrew)

[jgm]

Diagnosis: When makeSections creates the Divs dividing the document into sections, it sets the same classes on the Div as on the Header. But then line 1647 of src/Text/Pandoc/Writers.HTML,

              t <- addAttrs opts attr header'

ends up adding the attribute again.

The problem extends to all classes and key/value pairs:

echo '[Header 1 ("section",["foo","unnumbered"],[("key","val")]) [Str "1"]]' | pandoc -t html -f native
("section",["foo","unnumbered"],[("key","val")])
<h1 class="foo unnumbered" data-key="val" id="section" class="foo unnumbered" data-key="val">1</h1>