Issue #2670: urllib2.build_opener() failed when two handlers
derive the same default base class.

Backport of r62463.
diff --git a/Lib/test/test_urllib2.py b/Lib/test/test_urllib2.py
index 9934d37..96a7db9 100644
--- a/Lib/test/test_urllib2.py
+++ b/Lib/test/test_urllib2.py
@@ -1038,6 +1038,12 @@
         o = build_opener(urllib2.HTTPHandler())
         self.opener_has_handler(o, urllib2.HTTPHandler)
 
+        # Issue2670: multiple handlers sharing the same base class
+        class MyOtherHTTPHandler(urllib2.HTTPHandler): pass
+        o = build_opener(MyHTTPHandler, MyOtherHTTPHandler)
+        self.opener_has_handler(o, MyHTTPHandler)
+        self.opener_has_handler(o, MyOtherHTTPHandler)
+
     def opener_has_handler(self, opener, handler_class):
         for h in opener.handlers:
             if h.__class__ == handler_class:
diff --git a/Lib/urllib2.py b/Lib/urllib2.py
index 3578e7a..50d7aaf 100644
--- a/Lib/urllib2.py
+++ b/Lib/urllib2.py
@@ -447,14 +447,14 @@
                        FTPHandler, FileHandler, HTTPErrorProcessor]
     if hasattr(httplib, 'HTTPS'):
         default_classes.append(HTTPSHandler)
-    skip = []
+    skip = set()
     for klass in default_classes:
         for check in handlers:
             if isclass(check):
                 if issubclass(check, klass):
-                    skip.append(klass)
+                    skip.add(klass)
             elif isinstance(check, klass):
-                skip.append(klass)
+                skip.add(klass)
     for klass in skip:
         default_classes.remove(klass)