diff --git a/src/templates/project.html b/src/templates/project.html
index 6bc8854b..c1dd5f10 100644
--- a/src/templates/project.html
+++ b/src/templates/project.html
@@ -415,7 +415,6 @@ <h3 class="sidebar-card-title">📊 Portfolio Diversity</h3>
     </div>
   </footer>
 
-  <button id="scroll-top-btn" aria-label="Back to top" title="Back to top">↑</button>
   {% include 'partials/scroll_top_btn.html' %}
 
   <script>
diff --git a/tests/test_html_template_duplicate_ids.py b/tests/test_html_template_duplicate_ids.py
new file mode 100644
index 00000000..5823f70b
--- /dev/null
+++ b/tests/test_html_template_duplicate_ids.py
@@ -0,0 +1,32 @@
+# tests/test_html_template_duplicate_ids.py
+# Regression test for issue #1878: element IDs must be unique within a template.
+# Duplicate IDs are invalid HTML and break getElementById() lookups (dead UI).
+
+import glob
+import os
+import re
+
+ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+TEMPLATES_DIR = os.path.join(ROOT, "src", "templates")
+
+ID_RE = re.compile(r'\bid="([^"]+)"')
+
+
+def _template_files():
+    return sorted(glob.glob(os.path.join(TEMPLATES_DIR, "**", "*.html"), recursive=True))
+
+
+def test_no_duplicate_ids_in_templates():
+    """Each template may define an element ID only once."""
+    dupes = {}
+    for path in _template_files():
+        with open(path, encoding="utf-8") as f:
+            content = f.read()
+        ids = ID_RE.findall(content)
+        duplicated = {i for i in ids if ids.count(i) > 1}
+        if duplicated:
+            dupes[os.path.relpath(path, TEMPLATES_DIR)] = sorted(duplicated)
+    assert not dupes, (
+        "duplicate element IDs found in templates (issue #1878): "
+        f"{dupes}"
+    )