From 7e9be8dee323389fb90e73852be194cb7121ccc4 Mon Sep 17 00:00:00 2001
From: Krishan Mistry <krishan@krishanmistry.com>
Date: Tue, 29 Apr 2025 11:41:53 +0100
Subject: [PATCH] Warn on duplicate footnote definition and ignore subsequent
 definitions

---
 src/utils/mod.rs                              | 22 +++++++++++++++----
 tests/testsuite/markdown.rs                   | 20 +++++++++++++----
 .../footnotes/expected/footnotes.html         |  3 ---
 3 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/src/utils/mod.rs b/src/utils/mod.rs
index fc8584ee..0ca0482f 100644
--- a/src/utils/mod.rs
+++ b/src/utils/mod.rs
@@ -235,10 +235,10 @@ pub fn render_markdown_with_path(
     // `count` is the number of references to this footnote (used for multiple
     // linkbacks, and checking for unused footnotes).
     let mut footnote_numbers = HashMap::new();
-    // This is a list of (name, Vec<Event>)
+    // This is a map of name -> Vec<Event>
     // `name` is the name of the footnote.
     // The events list is the list of events needed to build the footnote definition.
-    let mut footnote_defs = Vec::new();
+    let mut footnote_defs = HashMap::new();
 
     // The following are used when currently processing a footnote definition.
     //
@@ -268,7 +268,16 @@ pub fn render_markdown_with_path(
                 Event::End(TagEnd::FootnoteDefinition) => {
                     let def_events = std::mem::take(&mut in_footnote);
                     let name = std::mem::take(&mut in_footnote_name);
-                    footnote_defs.push((name, def_events));
+
+                    if footnote_defs.contains_key(&name) {
+                        log::warn!(
+                            "footnote `{name}` in {} defined multiple times - \
+                             not updating to new definition",
+                            path.map_or_else(|| Cow::from("<unknown>"), |p| p.to_string_lossy())
+                        );
+                    } else {
+                        footnote_defs.insert(name, def_events);
+                    }
                     None
                 }
                 Event::FootnoteReference(name) => {
@@ -304,7 +313,12 @@ pub fn render_markdown_with_path(
     html::push_html(&mut body, events);
 
     if !footnote_defs.is_empty() {
-        add_footnote_defs(&mut body, path, footnote_defs, &footnote_numbers);
+        add_footnote_defs(
+            &mut body,
+            path,
+            footnote_defs.into_iter().collect(),
+            &footnote_numbers,
+        );
     }
 
     body
diff --git a/tests/testsuite/markdown.rs b/tests/testsuite/markdown.rs
index 537a8e06..c4bfbd07 100644
--- a/tests/testsuite/markdown.rs
+++ b/tests/testsuite/markdown.rs
@@ -17,10 +17,22 @@ fn custom_header_attributes() {
 // Test for a variety of footnote renderings.
 #[test]
 fn footnotes() {
-    BookTest::from_dir("markdown/footnotes").check_main_file(
-        "book/footnotes.html",
-        file!["markdown/footnotes/expected/footnotes.html"],
-    );
+    BookTest::from_dir("markdown/footnotes")
+        .run("build", |cmd| {
+            cmd.expect_stderr(str![[r#"
+[TIMESTAMP] [INFO] (mdbook::book): Book building has started
+[TIMESTAMP] [INFO] (mdbook::book): Running the html backend
+[TIMESTAMP] [WARN] (mdbook::utils): footnote `multiple-definitions` in <unknown> defined multiple times - not updating to new definition
+[TIMESTAMP] [WARN] (mdbook::utils): footnote `unused` in `<unknown>` is defined but not referenced
+[TIMESTAMP] [WARN] (mdbook::utils): footnote `multiple-definitions` in footnotes.md defined multiple times - not updating to new definition
+[TIMESTAMP] [WARN] (mdbook::utils): footnote `unused` in `footnotes.md` is defined but not referenced
+
+"#]]);
+        })
+        .check_main_file(
+            "book/footnotes.html",
+            file!["markdown/footnotes/expected/footnotes.html"],
+        );
 }
 
 // Basic table test.
diff --git a/tests/testsuite/markdown/footnotes/expected/footnotes.html b/tests/testsuite/markdown/footnotes/expected/footnotes.html
index 28c3d94e..8904cb06 100644
--- a/tests/testsuite/markdown/footnotes/expected/footnotes.html
+++ b/tests/testsuite/markdown/footnotes/expected/footnotes.html
@@ -40,9 +40,6 @@ With a reference inside.<sup class="footnote-reference" id="fr-1-2"><a href="#fo
 <li id="footnote-multiple-definitions">
 <p>This is the first definition of the footnote with tag multiple-definitions <a href="#fr-multiple-definitions-1">↩</a> <a href="#fr-multiple-definitions-2">↩2</a></p>
 </li>
-<li id="footnote-multiple-definitions">
-<p>This is the second definition of the footnote with tag multiple-definitions <a href="#fr-multiple-definitions-1">↩</a> <a href="#fr-multiple-definitions-2">↩2</a></p>
-</li>
 <li id="footnote-in-between">
 <p>Footnote between duplicates. <a href="#fr-in-between-1">↩</a></p>
 </li>