diff --git a/linkcheck/checker/cache.py b/linkcheck/checker/cache.py
index c1e3f9dc..f1680cfb 100644
--- a/linkcheck/checker/cache.py
+++ b/linkcheck/checker/cache.py
@@ -200,7 +200,7 @@ class Cache (linkcheck.lock.AssertLock):
             self.checked[key] = data
             # add all aliases also to checked cache to avoid recursion
             for key in url_data.aliases:
-                if key not in self.checked:
+                if key not in self.checked and key not in self.in_progress:
                     linkcheck.log.debug(linkcheck.LOG_CACHE,
                                         "Cache alias %r...", key)
                     self.checked[key] = data