diff --git a/website/build.py b/website/build.py
index e030da37..0583a464 100644
--- a/website/build.py
+++ b/website/build.py
@@ -277,6 +277,15 @@ def build(repo_root: Path) -> None:
     sponsors = parse_sponsors(readme_text)
 
     categories = [cat for g in parsed_groups for cat in g["categories"]]
+    cat_slugs = [cat["slug"] for cat in categories]
+    group_slugs = [g["slug"] for g in parsed_groups]
+    all_top_level_slugs = cat_slugs + group_slugs
+    duplicates = {s for s in all_top_level_slugs if all_top_level_slugs.count(s) > 1}
+    if duplicates:
+        raise ValueError(
+            f"slug collision in /categories/ namespace: {sorted(duplicates)}. "
+            "Rename a category or group so their slugs differ."
+        )
     total_entries = sum(c["entry_count"] for c in categories)
     entries = extract_entries(categories, parsed_groups)
     build_date = datetime.now(UTC)
diff --git a/website/tests/test_build.py b/website/tests/test_build.py
index 0f666fb4..f64d44a1 100644
--- a/website/tests/test_build.py
+++ b/website/tests/test_build.py
@@ -484,6 +484,27 @@ class TestBuild:
         # Expand content present
         assert "expand-content" in html
 
+    def test_build_fails_when_group_and_category_slug_collide(self, tmp_path):
+        readme = textwrap.dedent("""\
+            # T
+
+            ---
+
+            **Widgets**
+
+            ## Widgets
+
+            - [w1](https://example.com) - W.
+
+            # Contributing
+
+            Done.
+        """)
+        self._make_repo(tmp_path, readme)
+        import pytest
+        with pytest.raises(ValueError, match="slug collision"):
+            build(tmp_path)
+
     def test_index_contains_aligned_homepage_metadata(self, tmp_path):
         readme = (Path(__file__).parents[2] / "README.md").read_text(encoding="utf-8")
         (tmp_path / "README.md").write_text(readme, encoding="utf-8")