fix(satteri): Make heading-ids plugin idempotent

.changeset/heading-ids-no-duplicates.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/markdown-satteri': patch
+---
+
+Fixes headings being listed twice in a page's `headings` metadata when an integration (such as Starlight) assigns heading IDs with its own heading pass before adding anchor links

packages/markdown/satteri/src/satteri-processor.ts

@@ -117,6 +117,8 @@ export function makeFragmentNode(html: string): HastNode {
 
 export function createHeadingIdsPlugin(): HastPluginDefinition {
 	const slugger = new Slugger();
+	// Collect headings in a separate array so we can make this idempotent
+	const headings: MarkdownHeading[] = [];
 	return {
 		name: 'heading-ids',
 		element: {
@@ -130,7 +132,8 @@ export function createHeadingIdsPlugin(): HastPluginDefinition {
 				const existingId = node.properties?.id;
 				const slug = typeof existingId === 'string' ? existingId : slugger.slug(text);
 				const depth = Number.parseInt(node.tagName[1], 10);
-				astro?.headings.push({ depth, slug, text });
+				headings.push({ depth, slug, text });
+				if (astro) astro.headings = headings;
 				if (typeof existingId !== 'string') {
 					ctx.setProperty(node, 'id', slug);
 				}

packages/markdown/satteri/test/markdown.test.ts

@@ -74,6 +74,17 @@ describe('satteri markdown', () => {
 		assert.equal(seenId, 'hello-world');
 	});
 
+	it('does not duplicate headings when `satteriHeadingIdsPlugin()` runs as a user plugin too', async () => {
+		const processor = await createSatteriMarkdownProcessor({
+			hastPlugins: [satteriHeadingIdsPlugin()],
+		});
+		const { metadata } = await processor.render('## Some text\n\n## Some text');
+		assert.deepEqual(metadata.headings, [
+			{ depth: 2, slug: 'some-text', text: 'Some text' },
+			{ depth: 2, slug: 'some-text-1', text: 'Some text' },
+		]);
+	});
+
 	it('respects heading IDs set by a user hast plugin in both DOM and `headings`', async () => {
 		const setIdPlugin: HastPluginDefinition = {
 			name: 'set-heading-id',