diff -r d9c98730e2e8 Lib/argparse.py
--- a/Lib/argparse.py	Sat Jul 07 13:34:50 2012 +1000
+++ b/Lib/argparse.py	Sat Jul 07 17:05:00 2012 +0200
@@ -1465,9 +1465,16 @@
             raise ValueError(msg % self.conflict_handler)
 
     def _check_conflict(self, action):
+        confl_optionals = []
+
+        # mark duplicate positionals as conflicting
+        if not action.option_strings:  # optionals have option_strings
+            for _action in self._actions:
+                if (action.dest == _action.dest and
+                    not isinstance(action, _AppendAction)):
+                    confl_optionals.append((action.dest, _action))
 
         # find all options that conflict with this option
-        confl_optionals = []
         for option_string in action.option_strings:
             if option_string in self._option_string_actions:
                 confl_optional = self._option_string_actions[option_string]