Markdown writer: Fixed duplicated reference links

with `--reference-links` and `--reference-location=section`.
Also ensure that there are no empty link references `[]`.

Closes #3674.

1 files changed, 26 insertions, 0 deletions

diff --git a/test/command/3674.md b/test/command/3674.md
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+Make sure we don't get duplicate reference links, even with
+`--reference-location=section`.
+
+```
+% pandoc --reference-links -t markdown --reference-location=section --atx-headers
+# a
+
+![](a)
+
+# b
+
+![](b)
+
+^D
+# a
+
+![][1]
+
+  [1]: a
+
+# b
+
+![][2]
+
+  [2]: b
+```