Convert markdownlint library to an ECMAScript module, replace markdownlint-micromark with micromark, stop publishing (large) markdownlint-browser.js, see https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c for guidance.

lib/md024.mjs

@@ -0,0 +1,45 @@
+// @ts-check
+
+import { addErrorContext } from "../helpers/helpers.cjs";
+import { getHeadingLevel, getHeadingText } from "../helpers/micromark-helpers.cjs";
+import { filterByTypesCached } from "./cache.mjs";
+
+/** @type {import("./markdownlint.mjs").Rule} */
+export default {
+  "names": [ "MD024", "no-duplicate-heading" ],
+  "description": "Multiple headings with the same content",
+  "tags": [ "headings" ],
+  "parser": "micromark",
+  "function": function MD024(params, onError) {
+    const siblingsOnly = !!params.config.siblings_only || false;
+    const knownContents = [ null, [] ];
+    let lastLevel = 1;
+    let knownContent = knownContents[lastLevel];
+    for (const heading of filterByTypesCached([ "atxHeading", "setextHeading" ])) {
+      const headingText = getHeadingText(heading);
+      if (siblingsOnly) {
+        const newLevel = getHeadingLevel(heading);
+        while (lastLevel < newLevel) {
+          lastLevel++;
+          knownContents[lastLevel] = [];
+        }
+        while (lastLevel > newLevel) {
+          knownContents[lastLevel] = [];
+          lastLevel--;
+        }
+        knownContent = knownContents[newLevel];
+      }
+      // @ts-ignore
+      if (knownContent.includes(headingText)) {
+        addErrorContext(
+          onError,
+          heading.startLine,
+          headingText.trim()
+        );
+      } else {
+        // @ts-ignore
+        knownContent.push(headingText);
+      }
+    }
+  }
+};