Frame attributes are duplicated (e.g. .standout, .allowframebreaks)

[CGMossa]

Input:

---
title: A silly slide
subtitle: Look for the sequel!
---

# A circus!

## Another silly title

Not much to do about nothing.

## Epilogue {.standout}

Output:

\hypertarget{a-circus}{%
\section{A circus!}\label{a-circus}}

\begin{frame}{Another silly title}
\protect\hypertarget{another-silly-title}{}
Not much to do about nothing.
\end{frame}

\begin{frame}[standout,standout]{Epilogue} % This is the issue!
\protect\hypertarget{epilogue}{}
\end{frame}

The pandoc-call: pandoc -t beamer reports/silly_slide.md -o reports/silly_slide_md.md

Pandoc version:

pandoc.exe 2.9.1.1
Compiled with pandoc-types 1.20, texmath 0.12, skylighting 0.8.3
[redacted]

I've tried to detect the source of the issue, but couldn't find it. The template doesn't say much,
and the code that is concerned with this is here:

  let frameoptions = ["allowdisplaybreaks", "allowframebreaks", "fragile",
                      "b", "c", "t", "environment",
                      "label", "plain", "shrink", "standout",
                      "noframenumbering"]
  let optionslist = ["fragile" | fragile
                               , isNothing (lookup "fragile" kvs)
                               , "fragile" `notElem` classes] ++
                    [k | k <- classes, k `elem` frameoptions] ++
                    [k <> "=" <> v | (k,v) <- kvs, k `elem` frameoptions]

[miasma]

I updated pandoc recently (pandoc 2.9.1.1) and noticed that this change might cause issues with standout frames that contain some content other than the frame title. For example see #3007 (comment) for an example of a bit more complex document (one more line).

---
title: ...
subtitle: ...
---

## Standout frame {.standout}
Standout (works as expected)

## Normal frame
Normal (works as expected)

When running pandoc, the example code produces this new error:

$ pandoc -t beamer test.md -o test.pdf
Error producing PDF.
! Package keyval Error: standout undefined.

See the keyval package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.111 \end{frame}

I have lots of documents that rely on the old behavior (standout frames with various content).

[jgm]

@miasma why don't you open a new issue for this.
Please have a look at the latex pandoc is generating (you can use --verbose) and also suggest what you think it should be generating.