diff -r 7ea87e3229fd5c4eebd0 oauth2/__init__.py
@@ -385,386 +385,391 @@
        url_items = self._split_url_string(query).items()
        non_oauth_url_items = list([(k, v) for k, v in url_items  if not k.startswith('oauth_')])
        
-       items.extend(non_oauth_url_items)
+       for (key,value) in non_oauth_url_items:
+           if (key,value) not in items:
+               items.append((key,value))
    
        encoded_str = urllib.urlencode(sorted(items))