diff --git a/src/core/render/slugify.js b/src/core/render/slugify.js
index 6f1f6a7ee..825011779 100644
--- a/src/core/render/slugify.js
+++ b/src/core/render/slugify.js
@@ -23,7 +23,7 @@ export function slugify(str) {
     .replace(/^(\d)/, '_$1');
   let count = cache[slug];
 
-  count = Object.keys(cache).includes(slug) ? count + 1 : 0;
+  count = Object.prototype.hasOwnProperty.call(cache, slug) ? count + 1 : 0;
   cache[slug] = count;
 
   if (count) {
diff --git a/test/unit/render-util.test.js b/test/unit/render-util.test.js
index 9636c01e6..77cb0c0fd 100644
--- a/test/unit/render-util.test.js
+++ b/test/unit/render-util.test.js
@@ -167,6 +167,10 @@ describe('core/render/tpl', () => {
 });
 
 describe('core/render/slugify', () => {
+  beforeEach(() => {
+    slugify.clear();
+  });
+
   test('slugify()', () => {
     const htmlStrippedSlug = slugify(
       'Bla bla bla <svg aria-label="broken" class="broken" viewPort="0 0 1 1"><circle cx="0.5" cy="0.5"/></svg>',
@@ -218,4 +222,18 @@ describe('core/render/slugify', () => {
     );
     expect(markdownLinkSlug).toBe('_500-rc4-2026-03-11');
   });
+
+  test('slugify.clear() resets duplicate tracking', () => {
+    expect(slugify('duplicate')).toBe('duplicate');
+    expect(slugify('duplicate')).toBe('duplicate-1');
+
+    slugify.clear();
+
+    expect(slugify('duplicate')).toBe('duplicate');
+  });
+
+  test('slugify() handles inherited property names as new slugs', () => {
+    expect(slugify('constructor')).toBe('constructor');
+    expect(slugify('constructor')).toBe('constructor-1');
+  });
 });