diff -r 732d70746fc0 Lib/argparse.py
--- a/Lib/argparse.py	Sat May 19 16:58:45 2012 +0800
+++ b/Lib/argparse.py	Sat May 19 14:14:42 2012 +0200
@@ -1088,11 +1088,15 @@
             self._choices_actions.append(choice_action)
 
         # create the parser and add it to the map
+        if name in self._name_parser_map:
+            raise ArgumentError(self, "Subparser '%s' is already defined" % name)
         parser = self._parser_class(**kwargs)
         self._name_parser_map[name] = parser
 
         # make parser available under aliases also
         for alias in aliases:
+            if alias in self._name_parser_map:
+                raise ArgumentError(self, "Subparser '%s' is already defined" % alias)
             self._name_parser_map[alias] = parser
 
         return parser
diff -r 732d70746fc0 Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Sat May 19 16:58:45 2012 +0800
+++ b/Lib/test/test_argparse.py	Sat May 19 14:14:42 2012 +0200
@@ -2001,6 +2001,11 @@
                                     1 help
                 2                   2 help
             """))
+    def test_error_already_defined_subparser(self):
+        parser = ErrorRaisingArgumentParser()
+        subparsers = parser.add_subparsers()
+        subparsers.add_parser('1')
+        self.assertRaises(argparse.ArgumentError, subparsers.add_parser, '1')
 
 # ============
 # Groups tests