diff -r eba80326ba53 Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py	Sun Apr 05 19:26:29 2015 -0400
+++ b/Lib/test/test_warnings.py	Mon Apr 06 13:10:20 2015 +0300
@@ -247,6 +247,21 @@
             self.assertEqual(str(w[-1].message), text)
             self.assertTrue(w[-1].category is UserWarning)
 
+    def test_duplicate_filters(self):
+        with warnings_state(self.module):
+            initial_len = len(self.module.filters)
+            self.module.filterwarnings("error", category=UserWarning)
+            self.assertEqual(len(self.module.filters), initial_len + 1)
+            self.module.filterwarnings("error", category=UserWarning)
+            self.assertEqual(len(self.module.filters), initial_len + 1, "filterwarnings inserted duplicate filter")
+        with warnings_state(self.module):
+            initial_len = len(self.module.filters)
+            self.module.simplefilter("error", category=UserWarning)
+            self.assertEqual(len(self.module.filters), initial_len + 1)
+            self.module.simplefilter("error", category=UserWarning)
+            self.assertEqual(len(self.module.filters), initial_len + 1, "simplefilter inserted duplicate filter")
+
+ 
 class CFilterTests(FilterTests, unittest.TestCase):
     module = c_warnings
 
diff -r eba80326ba53 Lib/warnings.py
--- a/Lib/warnings.py	Sun Apr 05 19:26:29 2015 -0400
+++ b/Lib/warnings.py	Mon Apr 06 13:10:20 2015 +0300
@@ -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).
@@ -73,11 +74,12 @@
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
     item = (action, None, category, None, 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 resetwarnings():
     """Clear the list of warning filters, so that no filters are active."""