Fix implicit_header_references with duplicate headings.

Documentation says that when more than one heading has the same text,
an implicit reference `[Heading text][]` refers to the first one.
Previously pandoc linked to the last one instead. This patch
makes pandoc conform to the documented behavior.

See #8300.

src/Text/Pandoc/Readers/Markdown.hs

@@ -582,8 +582,9 @@ registerImplicitHeader raw attr@(ident, _, _)
   | T.null raw = return ()
   | otherwise = do
       let key = toKey $ "[" <> raw <> "]"
-      updateState $ \s ->
-        s { stateHeaderKeys = M.insert key (("#" <> ident,""), attr)
+      updateState $ \s ->  -- don't override existing headers
+        s { stateHeaderKeys = M.insertWith (\_new old -> old)
+                                     key (("#" <> ident,""), attr)
                                      (stateHeaderKeys s) }
 
 --

test/markdown-reader-more.native

@@ -357,7 +357,7 @@ Pandoc
       , Link
           ( "" , [] , [] )
           [ Str "My" , Space , Str "header" ]
-          ( "#my-header-1" , "" )
+          ( "#my-header" , "" )
       , Str "."
       ]
   , Para
@@ -367,7 +367,7 @@ Pandoc
       , Space
       , Str "to"
       , Space
-      , Link ( "" , [] , [] ) [ Str "it" ] ( "#my-header-1" , "" )
+      , Link ( "" , [] , [] ) [ Str "it" ] ( "#my-header" , "" )
       , Str "."
       ]
   , Para
@@ -378,7 +378,7 @@ Pandoc
       , Link
           ( "" , [] , [] )
           [ Str "case" , Space , Str "insensitive" ]
-          ( "#my-header-1" , "" )
+          ( "#my-header" , "" )
       , Str "."
       ]
   , Para