diff --git a/Lib/site.py b/Lib/site.py
index ba8603c..47196e4 100644
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -69,7 +69,7 @@
 
 def abs__file__():
     """Set all module' __file__ attribute to an absolute path"""
-    for m in sys.modules.values():
+    for m in set(sys.modules.values()):
         if hasattr(m, '__loader__'):
             continue   # don't mess with a PEP 302-supplied __file__
         try: