diff --git a/linkcheck/url.py b/linkcheck/url.py
index c3fad170..eaa620d7 100644
--- a/linkcheck/url.py
+++ b/linkcheck/url.py
@@ -588,6 +588,8 @@ def shorten_duplicate_content_url(url):
         url = url.split('#', 1)[0]
     if url.endswith('index.html'):
         return url[:-10]
+    if url.endswith('index.htm'):
+        return url[:-9]
     return url