From 064a1568a2ca4610257c69bb4dd81274f9c63424 Mon Sep 17 00:00:00 2001
From: Michael Ficarra <git@michael.ficarra.me>
Date: Tue, 7 Apr 2026 22:38:36 -0600
Subject: [PATCH 1/3] reject AO headers with duplicate parameter names

---
 src/header-parser.ts               | 13 +++++++++++--
 src/lint/rules/variable-use-def.ts |  3 +++
 test/lint.ts                       | 24 ++++++++++++++++++++++++
 3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/src/header-parser.ts b/src/header-parser.ts
index 9d97f6be..c2425219 100644
--- a/src/header-parser.ts
+++ b/src/header-parser.ts
@@ -105,6 +105,13 @@ export function parseHeader(headerText: string): ParsedHeaderOrFailure {
   let type: 'single-line' | 'multi-line';
   const params: Param[] = [];
   const optionalParams: Param[] = [];
+
+  function checkDuplicateParam(paramName: string, paramNameOffset: number) {
+    if (params.some(p => p.name === paramName) || optionalParams.some(p => p.name === paramName)) {
+      errors.push({ message: `duplicate parameter ${JSON.stringify(paramName)}`, offset: paramNameOffset });
+    }
+  }
+
   if (text[0] === '\n') {
     // multiline: parse for parameter types
     type = 'multi-line';
@@ -142,8 +149,9 @@ export function parseHeader(headerText: string): ParsedHeaderOrFailure {
         errors.push({ message: 'expected parameter name', offset });
         return { type: 'failure', errors };
       }
-      offset += match[0].length;
       const paramName = match[0].trimRight();
+      checkDuplicateParam(paramName, offset);
+      offset += match[0].length;
 
       ({ match, text } = eat(text, /^:+ */i));
       if (!match) {
@@ -216,8 +224,9 @@ export function parseHeader(headerText: string): ParsedHeaderOrFailure {
         errors.push({ message: 'expected parameter name', offset });
         return { type: 'failure', errors };
       }
-      offset += match[0].length;
       const paramName = match[0].trimRight();
+      checkDuplicateParam(paramName, offset);
+      offset += match[0].length;
 
       (optional ? optionalParams : params).push({
         name: paramName,
diff --git a/src/lint/rules/variable-use-def.ts b/src/lint/rules/variable-use-def.ts
index abb7b21b..fae76a07 100644
--- a/src/lint/rules/variable-use-def.ts
+++ b/src/lint/rules/variable-use-def.ts
@@ -144,8 +144,11 @@ export function checkVariableUsage(
       preceding.textContent != null
     ) {
       const isParameter = preceding.nodeName === 'H1';
+      const seen = new Set<string>();
       // `__` is for <del>_x_</del><ins>_y_</ins>, which has textContent `_x__y_`
       for (const name of preceding.textContent.matchAll(/(?<=\b|_)_([a-zA-Z0-9]+)_(?=\b|_)/g)) {
+        if (seen.has(name[1])) continue;
+        seen.add(name[1]);
         scope.declare(name[1], null, isParameter ? 'parameter' : undefined, !isParameter);
       }
     }
diff --git a/test/lint.ts b/test/lint.ts
index e3055a5f..da1069b4 100644
--- a/test/lint.ts
+++ b/test/lint.ts
@@ -424,6 +424,30 @@ describe('linting whole program', () => {
           </emu-clause>
       `);
     });
+
+    it('duplicate parameters', async () => {
+      await assertLint(
+        positioned`
+          <emu-clause id="example" type="abstract operation">
+            <h1>
+              Example (
+                _x_: ~foo~,
+                ${M}_x_: ~bar~,
+              ): ~foo~ or ~bar~
+            </h1>
+            <dl class="header"></dl>
+            <emu-alg>
+              1. Return _x_.
+            </emu-alg>
+          </emu-clause>
+        `,
+        {
+          ruleId: 'header-format',
+          nodeType: 'h1',
+          message: 'duplicate parameter "_x_"',
+        },
+      );
+    });
   });
 
   describe('closing tags', () => {

From 30e37e30eeb263aa28c4cc675714b98c23ac8df7 Mon Sep 17 00:00:00 2001
From: Michael Ficarra <git@michael.ficarra.me>
Date: Tue, 7 Apr 2026 23:59:41 -0600
Subject: [PATCH 2/3] fix lint

---
 src/header-parser.ts | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/header-parser.ts b/src/header-parser.ts
index c2425219..9d790534 100644
--- a/src/header-parser.ts
+++ b/src/header-parser.ts
@@ -108,7 +108,10 @@ export function parseHeader(headerText: string): ParsedHeaderOrFailure {
 
   function checkDuplicateParam(paramName: string, paramNameOffset: number) {
     if (params.some(p => p.name === paramName) || optionalParams.some(p => p.name === paramName)) {
-      errors.push({ message: `duplicate parameter ${JSON.stringify(paramName)}`, offset: paramNameOffset });
+      errors.push({
+        message: `duplicate parameter ${JSON.stringify(paramName)}`,
+        offset: paramNameOffset,
+      });
     }
   }
 

From fb3b6272bce7491090ed13ab74792ff5f3e2bb1f Mon Sep 17 00:00:00 2001
From: Kevin Gibbons <bakkot@gmail.com>
Date: Wed, 8 Apr 2026 09:48:54 -0700
Subject: [PATCH 3/3] add comment

---
 src/lint/rules/variable-use-def.ts | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/lint/rules/variable-use-def.ts b/src/lint/rules/variable-use-def.ts
index fae76a07..6f67a00b 100644
--- a/src/lint/rules/variable-use-def.ts
+++ b/src/lint/rules/variable-use-def.ts
@@ -147,6 +147,7 @@ export function checkVariableUsage(
       const seen = new Set<string>();
       // `__` is for <del>_x_</del><ins>_y_</ins>, which has textContent `_x__y_`
       for (const name of preceding.textContent.matchAll(/(?<=\b|_)_([a-zA-Z0-9]+)_(?=\b|_)/g)) {
+        // We avoid dealing with parameter re-declaration here because tracking location information is annoying. It's handled in `checkDuplicateParam` in header-parser instead.
         if (seen.has(name[1])) continue;
         seen.add(name[1]);
         scope.declare(name[1], null, isParameter ? 'parameter' : undefined, !isParameter);