feat: create no-duplicate-definitions

README.md

@@ -75,6 +75,7 @@ export default defineConfig([
 | :- | :- | :-: |
 | [`fenced-code-language`](./docs/rules/fenced-code-language.md) | Require languages for fenced code blocks | yes |
 | [`heading-increment`](./docs/rules/heading-increment.md) | Enforce heading levels increment by one | yes |
+| [`no-duplicate-definitions`](./docs/rules/no-duplicate-definitions.md) | Disallow duplicate definitions | yes |
 | [`no-duplicate-headings`](./docs/rules/no-duplicate-headings.md) | Disallow duplicate headings in the same document | no |
 | [`no-empty-images`](./docs/rules/no-empty-images.md) | Disallow empty images | yes |
 | [`no-empty-links`](./docs/rules/no-empty-links.md) | Disallow empty links | yes |

docs/rules/no-duplicate-definitions.md

@@ -0,0 +1,91 @@
+# no-duplicate-definitions
+
+Disallow duplicate definitions.
+
+## Background
+
+In Markdown, it's possible to define the same definition identifier multiple times. However, this is usually a mistake, as it can lead to unintended or incorrect link, image, and footnote references.
+
+Please note that this rule does not report definition-style comments. For example:
+
+```markdown
+[//]: # (This is a comment 1)
+[//]: <> (This is a comment 2)
+```
+
+## Rule Details
+
+> [!IMPORTANT] <!-- eslint-disable-line -- This should be fixed in https://github.com/eslint/markdown/issues/294 -->
+>
+> The `FootnoteDefinition` node is detected only when using `language` mode [`markdown/gfm`](/README.md#languages).
+
+This rule warns when `Definition` and `FootnoteDefinition` type identifiers are defined multiple times. Please note that this rule is **case-insensitive**, meaning `earth` and `Earth` are treated as the same identifier.
+
+Examples of **incorrect** code:
+
+```markdown
+<!-- eslint markdown/no-duplicate-definitions: "error" -->
+<!-- definition -->
+
+[mercury]: https://example.com/mercury/
+[mercury]: https://example.com/venus/
+
+[earth]: https://example.com/earth/
+[Earth]: https://example.com/mars/ 
+
+<!-- footnote definition -->
+
+[^mercury]: Hello, Mercury!
+[^mercury]: Hello, Venus!
+
+[^earth]: Hello, Earth!
+[^Earth]: Hello, Mars!
+```
+
+Examples of **correct** code:
+
+```markdown
+<!-- eslint markdown/no-duplicate-definitions: "error" -->
+<!-- definition -->
+
+[mercury]: https://example.com/mercury/
+[venus]: https://example.com/venus/
+
+<!-- footnote definition -->
+
+[^mercury]: Hello, Mercury!
+[^venus]: Hello, Venus!
+
+<!-- definition-style comment -->
+
+[//]: # (This is a comment 1)
+[//]: <> (This is a comment 2)
+```
+
+## Options
+
+The following options are available on this rule:
+
+- `allowDefinitions: Array<string>` - when specified, duplicate definitions are allowed if they match one of the identifiers in this array. This is useful for ignoring definitions that are intentionally duplicated. (default: `["//"]`)
+
+    Examples of **correct** code when configured as `"no-duplicate-definitions: ["error", { allowDefinitions: ["mercury"] }]`:
+
+    ```markdown
+    <!-- eslint markdown/no-duplicate-definitions: ["error", { allowDefinitions: ["mercury"] }] -->
+    [mercury]: https://example.com/mercury/
+    [mercury]: https://example.com/venus/
+    ```
+
+- `allowFootnoteDefinitions: Array<string>` - when specified, duplicate footnote definitions are allowed if they match one of the identifiers in this array. This is useful for ignoring footnote definitions that are intentionally duplicated. (default: `[]`)
+
+    Examples of **correct** code when configured as `"no-duplicate-definitions: ["error", { allowFootnoteDefinitions: ["mercury"] }]`:
+
+    ```markdown
+    <!-- eslint markdown/no-duplicate-definitions: ["error", { allowFootnoteDefinitions: ["mercury"] }] -->
+    [^mercury]: Hello, Mercury!
+    [^mercury]: Hello, Venus!
+    ```
+
+## When Not to Use It
+
+If you are using a different style of definition comments, or not concerned with duplicate definitions, you can safely disable this rule.

src/rules/no-duplicate-definitions.js

@@ -0,0 +1,106 @@
+/**
+ * @fileoverview Rule to prevent duplicate definitions in Markdown.
+ * @author 루밀LuMir(lumirlumir)
+ */
+
+//-----------------------------------------------------------------------------
+// Type Definitions
+//-----------------------------------------------------------------------------
+
+/**
+ * @typedef {import("../types.ts").MarkdownRuleDefinition<{ RuleOptions: [{ allowDefinitions: string[], allowFootnoteDefinitions: string[]; }]; }>}
+ * NoDuplicateDefinitionsRuleDefinition
+ */
+
+//-----------------------------------------------------------------------------
+// Rule Definition
+//-----------------------------------------------------------------------------
+
+/** @type {NoDuplicateDefinitionsRuleDefinition} */
+export default {
+	meta: {
+		type: "problem",
+
+		docs: {
+			recommended: true,
+			description: "Disallow duplicate definitions",
+			url: "https://github.com/eslint/markdown/blob/main/docs/rules/no-duplicate-definitions.md",
+		},
+
+		messages: {
+			duplicateDefinition: "Unexpected duplicate definition found.",
+			duplicateFootnoteDefinition:
+				"Unexpected duplicate footnote definition found.",
+		},
+
+		schema: [
+			{
+				type: "object",
+				properties: {
+					allowDefinitions: {
+						type: "array",
+						items: {
+							type: "string",
+						},
+						uniqueItems: true,
+					},
+					allowFootnoteDefinitions: {
+						type: "array",
+						items: {
+							type: "string",
+						},
+						uniqueItems: true,
+					},
+				},
+				additionalProperties: false,
+			},
+		],
+
+		defaultOptions: [
+			{
+				allowDefinitions: ["//"],
+				allowFootnoteDefinitions: [],
+			},
+		],
+	},
+
+	create(context) {
+		const [{ allowDefinitions, allowFootnoteDefinitions }] =
+			context.options;
+
+		const definitions = new Set();
+		const footnoteDefinitions = new Set();
+
+		return {
+			definition(node) {
+				if (allowDefinitions.includes(node.identifier)) {
+					return;
+				}
+
+				if (definitions.has(node.identifier)) {
+					context.report({
+						node,
+						messageId: "duplicateDefinition",
+					});
+				} else {
+					definitions.add(node.identifier);
+				}
+			},
+
+			footnoteDefinition(node) {
+				if (allowFootnoteDefinitions.includes(node.identifier)) {
+					return;
+				}
+
+				if (footnoteDefinitions.has(node.identifier)) {
+					context.report({
+						node,
+						messageId: "duplicateFootnoteDefinition",
+					});
+				} else {
+					footnoteDefinitions.add(node.identifier);
+				}
+			},
+		};
+	},
+};