diff --git a/src/validators/duplicates.js b/src/validators/duplicates.js
index 926d5da..33ee80d 100644
--- a/src/validators/duplicates.js
+++ b/src/validators/duplicates.js
@@ -15,9 +15,21 @@ export function validateAll(files) {
     if (!CHECKED_DIRS.includes(topDir)) continue;
 
     const filename = path.basename(relPath);
-    if (filename === 'index.md' || filename === 'index.mdx') continue;
+    const isIndex = filename === 'index.md' || filename === 'index.mdx';
+
+    // A folder's index.md is a category page whose slug is the folder name, so
+    // it competes for a slug just like a `<slug>.md` file does. Any other file's
+    // slug is its own filename. Skip a section-root index (docs/index.md,
+    // reference/index.md) — it has no competing leaf slug.
+    let slug;
+    if (isIndex) {
+      const parent = path.basename(path.dirname(relPath));
+      if (parent === topDir) continue;
+      slug = parent;
+    } else {
+      slug = filename.replace(/\.(md|mdx)$/, '');
+    }
 
-    const slug = filename.replace(/\.(md|mdx)$/, '');
     const key = `${topDir}:${slug}`;
     if (!slugMap.has(key)) slugMap.set(key, []);
     slugMap.get(key).push(relPath);
diff --git a/test/duplicates.test.js b/test/duplicates.test.js
index 436ae17..10eaf8a 100644
--- a/test/duplicates.test.js
+++ b/test/duplicates.test.js
@@ -13,3 +13,36 @@ test('same slug within docs (different subdirs) is flagged', () => {
   assert.match(res[0].message, /Duplicate slug: "intro"/);
   assert.equal(res[0].severity, 'error');
 });
+
+test('a page slug colliding with a category folder (folder/index.md) is flagged', () => {
+  const res = validateAll([
+    'reference/API/Other/guides.md',
+    'reference/API/Other/guides/index.md',
+  ]);
+  assert.ok(Array.isArray(res) && res.length >= 1, 'expected a duplicate-slug error');
+  assert.match(res[0].message, /Duplicate slug: "guides"/);
+});
+
+test('category-folder slug collisions are section-wide, not per-folder', () => {
+  // A page in one subdir and a category folder in another, same section.
+  const res = validateAll([
+    'reference/A/guides.md',
+    'reference/B/guides/index.md',
+  ]);
+  assert.ok(Array.isArray(res) && res.length >= 1);
+  assert.match(res[0].message, /Duplicate slug: "guides"/);
+});
+
+test('a section-root index page is not treated as a competing slug', () => {
+  // reference/index.md is the section landing; it must not collide with anything.
+  const res = validateAll(['reference/index.md', 'reference/foo.md']);
+  assert.equal(res, null);
+});
+
+test('a category folder slug in reference does not collide with the same slug in docs', () => {
+  const res = validateAll([
+    'docs/guides.md',
+    'reference/API/Other/guides/index.md',
+  ]);
+  assert.equal(res, null, 'docs and reference are separate namespaces');
+});