From cc66c0f810f3a39e21c5555fc3bf62452765e661 Mon Sep 17 00:00:00 2001
From: NullVoxPopuli <199018+NullVoxPopuli@users.noreply.github.com>
Date: Tue, 10 Mar 2026 18:27:13 -0400
Subject: [PATCH] Extract rule: template-no-duplicate-attributes

---
 README.md                                     |   1 +
 .../rules/template-no-duplicate-attributes.md |  59 ++++++
 lib/rules/template-no-duplicate-attributes.js |  85 +++++++++
 .../rules/template-no-duplicate-attributes.js | 179 ++++++++++++++++++
 4 files changed, 324 insertions(+)
 create mode 100644 docs/rules/template-no-duplicate-attributes.md
 create mode 100644 lib/rules/template-no-duplicate-attributes.js
 create mode 100644 tests/lib/rules/template-no-duplicate-attributes.js

diff --git a/README.md b/README.md
index 4709a8b15c..908553fc75 100644
--- a/README.md
+++ b/README.md
@@ -216,6 +216,7 @@ rules in templates can be disabled with eslint directives with mustache or html
 | [template-no-class-bindings](docs/rules/template-no-class-bindings.md)                                       | disallow passing classBinding or classNameBindings as arguments in templates |    |    |    |
 | [template-no-curly-component-invocation](docs/rules/template-no-curly-component-invocation.md)               | disallow curly component invocation, use angle bracket syntax instead        |    |    |    |
 | [template-no-debugger](docs/rules/template-no-debugger.md)                                                   | disallow {{debugger}} in templates                                           |    |    |    |
+| [template-no-duplicate-attributes](docs/rules/template-no-duplicate-attributes.md)                           | disallow duplicate attribute names in templates                              |    | 🔧 |    |
 | [template-no-duplicate-id](docs/rules/template-no-duplicate-id.md)                                           | disallow duplicate id attributes                                             |    |    |    |
 | [template-no-dynamic-subexpression-invocations](docs/rules/template-no-dynamic-subexpression-invocations.md) | disallow dynamic subexpression invocations                                   |    |    |    |
 | [template-no-element-event-actions](docs/rules/template-no-element-event-actions.md)                         | disallow element event actions (use {{on}} modifier instead)                 |    |    |    |
diff --git a/docs/rules/template-no-duplicate-attributes.md b/docs/rules/template-no-duplicate-attributes.md
new file mode 100644
index 0000000000..2fec029aa6
--- /dev/null
+++ b/docs/rules/template-no-duplicate-attributes.md
@@ -0,0 +1,59 @@
+# ember/template-no-duplicate-attributes
+
+🔧 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).
+
+<!-- end auto-generated rule header -->
+
+Disallows duplicate attribute names in templates.
+
+Duplicate attributes on the same element can lead to unexpected behavior and are often a mistake.
+
+## Rule Details
+
+This rule disallows duplicate attributes on HTML elements, components, and helpers.
+
+## Examples
+
+Examples of **incorrect** code for this rule:
+
+```gjs
+<template>
+  <div class="foo" class="bar"></div>
+</template>
+```
+
+```gjs
+<template>
+  <input type="text" disabled type="email" />
+</template>
+```
+
+```gjs
+<template>
+  {{helper foo="bar" foo="baz"}}
+</template>
+```
+
+Examples of **correct** code for this rule:
+
+```gjs
+<template>
+  <div class="foo bar"></div>
+</template>
+```
+
+```gjs
+<template>
+  <input type="email" disabled />
+</template>
+```
+
+```gjs
+<template>
+  {{helper foo="bar" baz="qux"}}
+</template>
+```
+
+## References
+
+- [eslint-plugin-ember template-no-duplicate-attributes](https://github.com/ember-cli/eslint-plugin-ember/blob/master/docs/rules/template-no-duplicate-attributes.md)
diff --git a/lib/rules/template-no-duplicate-attributes.js b/lib/rules/template-no-duplicate-attributes.js
new file mode 100644
index 0000000000..750b3d9a86
--- /dev/null
+++ b/lib/rules/template-no-duplicate-attributes.js
@@ -0,0 +1,85 @@
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'disallow duplicate attribute names in templates',
+      category: 'Best Practices',
+      url: 'https://github.com/ember-cli/eslint-plugin-ember/tree/master/docs/rules/template-no-duplicate-attributes.md',
+      templateMode: 'both',
+    },
+    fixable: 'code',
+    schema: [],
+    messages: {
+      duplicateElement: "Duplicate attribute '{{name}}' found in the Element.",
+      duplicateBlock: "Duplicate attribute '{{name}}' found in the BlockStatement.",
+      duplicateMustache: "Duplicate attribute '{{name}}' found in the MustacheStatement.",
+      duplicateSubExpr: "Duplicate attribute '{{name}}' found in the SubExpression.",
+    },
+    originallyFrom: {
+      name: 'ember-template-lint',
+      rule: 'lib/rules/no-duplicate-attributes.js',
+      docs: 'docs/rule/no-duplicate-attributes.md',
+      tests: 'test/unit/rules/no-duplicate-attributes-test.js',
+    },
+  },
+
+  create(context) {
+    function checkForDuplicates(node, attributes, identifier, messageId) {
+      if (!attributes || attributes.length < 2) {
+        return;
+      }
+
+      const seen = new Map();
+
+      for (const attr of attributes) {
+        const key = attr[identifier];
+        if (seen.has(key)) {
+          context.report({
+            node: attr,
+            messageId,
+            data: { name: key },
+            fix(fixer) {
+              // Remove the duplicate attribute including preceding whitespace
+              const sourceCode = context.sourceCode;
+              const text = sourceCode.getText();
+              const attrStart = attr.range[0];
+              const attrEnd = attr.range[1];
+
+              // Look for whitespace before the attribute
+              let removeStart = attrStart;
+              while (removeStart > 0 && /\s/.test(text[removeStart - 1])) {
+                removeStart--;
+              }
+
+              return fixer.removeRange([removeStart, attrEnd]);
+            },
+          });
+        } else {
+          seen.set(key, attr);
+        }
+      }
+    }
+
+    return {
+      GlimmerElementNode(node) {
+        checkForDuplicates(node, node.attributes, 'name', 'duplicateElement');
+      },
+
+      GlimmerBlockStatement(node) {
+        const attributes = node.hash?.pairs || [];
+        checkForDuplicates(node, attributes, 'key', 'duplicateBlock');
+      },
+
+      GlimmerMustacheStatement(node) {
+        const attributes = node.hash?.pairs || [];
+        checkForDuplicates(node, attributes, 'key', 'duplicateMustache');
+      },
+
+      GlimmerSubExpression(node) {
+        const attributes = node.hash?.pairs || [];
+        checkForDuplicates(node, attributes, 'key', 'duplicateSubExpr');
+      },
+    };
+  },
+};
diff --git a/tests/lib/rules/template-no-duplicate-attributes.js b/tests/lib/rules/template-no-duplicate-attributes.js
new file mode 100644
index 0000000000..e1ea5d6de4
--- /dev/null
+++ b/tests/lib/rules/template-no-duplicate-attributes.js
@@ -0,0 +1,179 @@
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/template-no-duplicate-attributes');
+const RuleTester = require('eslint').RuleTester;
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({
+  parser: require.resolve('ember-eslint-parser'),
+  parserOptions: { ecmaVersion: 2022, sourceType: 'module' },
+});
+
+ruleTester.run('template-no-duplicate-attributes', rule, {
+  valid: [
+    `<template>
+      <div class="foo" id="bar"></div>
+    </template>`,
+    `<template>
+      <button type="button" disabled></button>
+    </template>`,
+    `<template>
+      {{helper arg1="a" arg2="b"}}
+    </template>`,
+    `<template>
+      {{#each items as |item|}}
+        {{item}}
+      {{/each}}
+    </template>`,
+
+    '<template>{{my-component firstName=firstName lastName=lastName}}</template>',
+    '<template> {{fullName}}</template>',
+    '<template><a class="btn">{{btnLabel}}</a></template>',
+    '<template>{{employee-profile employee=(hash firstName=firstName lastName=lastName age=age)}}</template>',
+    '<template>{{employee-profile employee=(hash fullName=(hash firstName=firstName lastName=lastName) age=age)}}</template>',
+
+    // Block form with params (no duplicates)
+    '<template>{{#my-component firstName=firstName lastName=lastName as |fullName|}}{{fullName}}{{/my-component}}</template>',
+  ],
+
+  invalid: [
+    {
+      code: `<template>
+        <div class="foo" class="bar"></div>
+      </template>`,
+      output: `<template>
+        <div class="foo"></div>
+      </template>`,
+      errors: [
+        {
+          message: "Duplicate attribute 'class' found in the Element.",
+          type: 'GlimmerAttrNode',
+        },
+      ],
+    },
+    {
+      code: `<template>
+        <input type="text" disabled type="email" />
+      </template>`,
+      output: `<template>
+        <input type="text" disabled />
+      </template>`,
+      errors: [
+        {
+          message: "Duplicate attribute 'type' found in the Element.",
+          type: 'GlimmerAttrNode',
+        },
+      ],
+    },
+    {
+      code: `<template>
+        {{helper foo="bar" foo="baz"}}
+      </template>`,
+      output: `<template>
+        {{helper foo="bar"}}
+      </template>`,
+      errors: [
+        {
+          message: "Duplicate attribute 'foo' found in the MustacheStatement.",
+          type: 'GlimmerHashPair',
+        },
+      ],
+    },
+    {
+      code: `<template>
+        {{#if condition key="a" key="b"}}
+          content
+        {{/if}}
+      </template>`,
+      output: `<template>
+        {{#if condition key="a"}}
+          content
+        {{/if}}
+      </template>`,
+      errors: [
+        {
+          message: "Duplicate attribute 'key' found in the BlockStatement.",
+          type: 'GlimmerHashPair',
+        },
+      ],
+    },
+
+    {
+      code: '<template>{{my-component firstName=firstName lastName=lastName firstName=firstName}}</template>',
+      output: '<template>{{my-component firstName=firstName lastName=lastName}}</template>',
+      errors: [{ messageId: 'duplicateMustache', data: { name: 'firstName' } }],
+    },
+    {
+      code: '<template>{{#my-component firstName=firstName lastName=lastName firstName=firstName as |fullName|}}{{/my-component}}</template>',
+      output:
+        '<template>{{#my-component firstName=firstName lastName=lastName as |fullName|}}{{/my-component}}</template>',
+      errors: [{ messageId: 'duplicateBlock', data: { name: 'firstName' } }],
+    },
+    {
+      code: '<template><a class="btn" class="btn">{{btnLabel}}</a></template>',
+      output: '<template><a class="btn">{{btnLabel}}</a></template>',
+      errors: [{ messageId: 'duplicateElement', data: { name: 'class' } }],
+    },
+    {
+      code: '<template>{{employee-profile employee=(hash firstName=firstName lastName=lastName age=age firstName=firstName)}}</template>',
+      output:
+        '<template>{{employee-profile employee=(hash firstName=firstName lastName=lastName age=age)}}</template>',
+      errors: [{ messageId: 'duplicateSubExpr', data: { name: 'firstName' } }],
+    },
+    {
+      code: '<template>{{employee-profile employee=(hash fullName=(hash firstName=firstName lastName=lastName firstName=firstName) age=age)}}</template>',
+      output:
+        '<template>{{employee-profile employee=(hash fullName=(hash firstName=firstName lastName=lastName) age=age)}}</template>',
+      errors: [{ messageId: 'duplicateSubExpr', data: { name: 'firstName' } }],
+    },
+  ],
+});
+
+const hbsRuleTester = new RuleTester({
+  parser: require.resolve('ember-eslint-parser/hbs'),
+  parserOptions: { ecmaVersion: 2022, sourceType: 'module' },
+});
+
+hbsRuleTester.run('template-no-duplicate-attributes (hbs)', rule, {
+  valid: [
+    '{{my-component firstName=firstName lastName=lastName}}',
+    '{{#my-component firstName=firstName  lastName=lastName as |fullName|}} {{fullName}}{{/my-component}}',
+    '<a class="btn">{{btnLabel}}</a>',
+    '{{employee-profile employee=(hash firstName=firstName lastName=lastName age=age)}}',
+    '{{employee-profile employee=(hash fullName=(hash firstName=firstName lastName=lastName) age=age)}}',
+  ],
+  invalid: [
+    {
+      code: '{{my-component firstName=firstName lastName=lastName firstName=firstName}}',
+      output: '{{my-component firstName=firstName lastName=lastName}}',
+      errors: [{ messageId: 'duplicateMustache', data: { name: 'firstName' } }],
+    },
+    {
+      code: '{{#my-component firstName=firstName  lastName=lastName firstName=firstName as |fullName|}} {{fullName}}{{/my-component}}',
+      output:
+        '{{#my-component firstName=firstName  lastName=lastName as |fullName|}} {{fullName}}{{/my-component}}',
+      errors: [{ messageId: 'duplicateBlock', data: { name: 'firstName' } }],
+    },
+    {
+      code: '<a class="btn" class="btn">{{btnLabel}}</a>',
+      output: '<a class="btn">{{btnLabel}}</a>',
+      errors: [{ messageId: 'duplicateElement', data: { name: 'class' } }],
+    },
+    {
+      code: '{{employee-profile employee=(hash firstName=firstName lastName=lastName age=age firstName=firstName)}}',
+      output: '{{employee-profile employee=(hash firstName=firstName lastName=lastName age=age)}}',
+      errors: [{ messageId: 'duplicateSubExpr', data: { name: 'firstName' } }],
+    },
+    {
+      code: '{{employee-profile employee=(hash fullName=(hash firstName=firstName lastName=lastName firstName=firstName) age=age)}}',
+      output:
+        '{{employee-profile employee=(hash fullName=(hash firstName=firstName lastName=lastName) age=age)}}',
+      errors: [{ messageId: 'duplicateSubExpr', data: { name: 'firstName' } }],
+    },
+  ],
+});