diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -254,16 +254,49 @@ class FilterTests(BaseTest):
 
         L = [("default",X(),UserWarning,X(),0) for i in range(2)]
         with original_warnings.catch_warnings(record=True,
                 module=self.module) as w:
             self.module.filters = L
             self.module.warn_explicit(UserWarning("b"), None, "f.py", 42)
             self.assertEqual(str(w[-1].message), "b")
 
+    def test_filterwarnings_duplicate_filters(self):
+        with original_warnings.catch_warnings(module=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("ignore", category=UserWarning)
+            self.module.filterwarnings("error", category=UserWarning)
+            self.assertEqual(
+                len(self.module.filters), initial_len + 2,
+                "filterwarnings inserted duplicate filter"
+            )
+            self.assertEqual(
+                self.module.filters[0][0], "error",
+                "filterwarnings did not promote filter to "
+                "the beginning of list"
+            )
+
+    def test_simplefilter_duplicate_filters(self):
+        with original_warnings.catch_warnings(module=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("ignore", category=UserWarning)
+            self.module.simplefilter("error", category=UserWarning)
+            self.assertEqual(
+                len(self.module.filters), initial_len + 2,
+                "simplefilter inserted duplicate filter"
+            )
+            self.assertEqual(
+                self.module.filters[0][0], "error",
+                "simplefilter did not promote filter to the beginning of list"
+            )
+
 class CFilterTests(FilterTests, unittest.TestCase):
     module = c_warnings
 
 class PyFilterTests(FilterTests, unittest.TestCase):
     module = py_warnings
 
 
 class WarnTests(BaseTest):
diff --git a/Lib/warnings.py b/Lib/warnings.py
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -47,16 +47,23 @@ def filterwarnings(action, message="", c
     assert isinstance(message, str), "message must be a string"
     assert isinstance(category, type), "category must be a class"
     assert issubclass(category, Warning), "category must be a Warning subclass"
     assert isinstance(module, str), "module must be a string"
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
     item = (action, re.compile(message, re.I), category,
             re.compile(module), lineno)
+
+    # Remove possible duplicate filters, so new one will be placed
+    # in correct place
+    try:
+        filters.remove(item)
+    except ValueError:
+        pass
     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).
@@ -68,16 +75,23 @@ def simplefilter(action, category=Warnin
     'lineno' -- an integer line number, 0 matches all warnings
     'append' -- if true, append to the list of filters
     """
     assert action in ("error", "ignore", "always", "default", "module",
                       "once"), "invalid action: %r" % (action,)
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
     item = (action, None, category, None, lineno)
+
+    # Remove possible duplicate filters, so new one will be placed
+    # in correct place
+    try:
+        filters.remove(item)
+    except ValueError:
+        pass
     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."""
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -61,16 +61,19 @@ Core and Builtins
 - Issue #24044: Fix possible null pointer dereference in list.sort in out of
   memory conditions.
 
 - Issue #21354: PyCFunction_New function is exposed by python DLL again.
 
 Library
 -------
 
+- Issue #18383: Avoid creating duplicate filters when using filterwarnings
+  and simplefilter.  Patch by Alex Shkop.
+
 - Issue #23319: Fix ctypes.BigEndianStructure, swap correctly bytes. Patch
   written by Matthieu Gautier.
 
 - Issue #23254: Document how to close the TCPServer listening socket.
   Patch from Martin Panter.
 
 - Issue #19450: Update Windows and OS X installer builds to use SQLite 3.8.11.