Index: Lib/ConfigParser.py
===================================================================
--- Lib/ConfigParser.py	(revision 68546)
+++ Lib/ConfigParser.py	(working copy)
@@ -215,13 +215,14 @@
 
 
 class RawConfigParser:
-    def __init__(self, defaults=None, dict_type=dict):
+    def __init__(self, defaults=None, dict_type=dict, unique_sects=False):
         self._dict = dict_type
         self._sections = self._dict()
         self._defaults = self._dict()
         if defaults:
             for key, value in defaults.items():
                 self._defaults[self.optionxform(key)] = value
+        self._unique_sects = unique_sects
 
     def defaults(self):
         return self._defaults
@@ -468,7 +469,10 @@
                 if mo:
                     sectname = mo.group('header')
                     if sectname in self._sections:
-                        cursect = self._sections[sectname]
+                        if self._unique_sects:
+                            raise DuplicateSectionError(sectname)
+                        else:
+                            cursect = self._sections[sectname]
                     elif sectname == DEFAULTSECT:
                         cursect = self._defaults
                     else: