From edd48fceb14a9b4df27d35e2ac5face06a498a67 Mon Sep 17 00:00:00 2001
From: John MacFarlane <jgm@berkeley.edu>
Date: Tue, 22 Jul 2025 20:43:48 -0700
Subject: Djot writer: fix duplicate attributes before section headings.

Closes #10984.
---
 src/Text/Pandoc/Writers/Djot.hs |  9 ++++++---
 test/command/10984.md           | 16 ++++++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)
 create mode 100644 test/command/10984.md

diff --git a/src/Text/Pandoc/Writers/Djot.hs b/src/Text/Pandoc/Writers/Djot.hs
index 45ea729eb..c7e846735 100644
--- a/src/Text/Pandoc/Writers/Djot.hs
+++ b/src/Text/Pandoc/Writers/Djot.hs
@@ -100,7 +100,8 @@ blockToDjot (BlockQuote bls) = D.blockQuote <$> blocksToDjot bls
 blockToDjot (Header lev attr ils) =
   fmap (D.addAttr (toDjotAttr attr)) . D.heading lev <$> inlinesToDjot ils
 blockToDjot HorizontalRule = pure D.thematicBreak
-blockToDjot (Div (ident,"section":cls,kvs) bls@(Header _ _ ils : _)) = do
+blockToDjot (Div (ident,"section":cls,kvs)
+              (Header lev (_,hcls,hkvs) ils : bls)) = do
   ilsBs <- D.inlinesToByteString <$> inlinesToDjot ils
   let ident' = toIdentifier ilsBs
   let label = D.normalizeLabel ilsBs
@@ -112,8 +113,10 @@ blockToDjot (Div (ident,"section":cls,kvs) bls@(Header _ _ ils : _)) = do
   fmap
     (D.addAttr (toDjotAttr (if autoid then "" else ident,
                          filter (/= "section") cls,
-                         filter (\(k,_) -> k /= "wrapper") kvs))) . D.section
-     <$> blocksToDjot bls
+                         filter (\(k,_) -> k /= "wrapper") kvs) <>
+                toDjotAttr ("", [c | c <- hcls, c `notElem` cls], hkvs)))
+         . D.section
+     <$> blocksToDjot (Header lev mempty ils : bls)
 blockToDjot (Div attr@(ident,cls,kvs) bls)
   | Just "1" <- lookup "wrapper" kvs
     = fmap (D.addAttr
diff --git a/test/command/10984.md b/test/command/10984.md
new file mode 100644
index 000000000..b718d959d
--- /dev/null
+++ b/test/command/10984.md
@@ -0,0 +1,16 @@
+ ```
+ % pandoc -f html -t djot
+<h1 id="foo" class="a b">Hi</hi>
+^D
+{#foo .a .b}
+# Hi
+```
+
+In this one the id is suppressed by the djot writer because
+the same one would be automatically generated by the djot reader:
+```
+% pandoc -f html -t djot
+<h2 id="Introduction">Introduction</h2>
+^D
+## Introduction
+```
-- 
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