diff -r 9ff477cd79da Lib/warnings.py
--- a/Lib/warnings.py	Mon Mar 02 07:41:00 2015 +0200
+++ b/Lib/warnings.py	Mon Mar 02 17:13:27 2015 +0200
@@ -52,11 +52,12 @@
            "lineno must be an int >= 0"
     item = (action, re.compile(message, re.I), category,
             re.compile(module), lineno)
-    if append:
-        filters.append(item)
-    else:
-        filters.insert(0, item)
-    _filters_mutated()
+    if item not in filters:
+        if append:
+            filters.append(item)
+        else:
+            filters.insert(0, item)
+        _filters_mutated()
 
 def simplefilter(action, category=Warning, lineno=0, append=False):
     """Insert a simple entry into the list of warnings filters (at the front).