Index: Lib/socket.py
===================================================================
--- Lib/socket.py	(Revision 58966)
+++ Lib/socket.py	(Arbeitskopie)
@@ -111,10 +111,10 @@
 
 
 # True if os.dup() can duplicate socket descriptors.
-# (On Windows at least, os.dup only works on files)
 _can_dup_socket = hasattr(_socket.socket, "dup")
 
-if _can_dup_socket:
+# On Windows _socket.socket.dup() doesn't accept a file descriptor
+if _can_dup_socket and os.name != 'nt':
     def fromfd(fd, family=AF_INET, type=SOCK_STREAM, proto=0):
         nfd = os.dup(fd)
         return socket(family, type, proto, fileno=nfd)
Index: Lib/test/test_socket.py
===================================================================
--- Lib/test/test_socket.py	(Revision 58966)
+++ Lib/test/test_socket.py	(Arbeitskopie)
@@ -566,7 +566,7 @@
     def testFromFd(self):
         # Testing fromfd()
         if not hasattr(socket, "fromfd"):
-            return # On Windows, this doesn't exist
+            return # On BeOS, OS/2 and RiscOS this doesn't exist
         fd = self.cli_conn.fileno()
         sock = socket.fromfd(fd, socket.AF_INET, socket.SOCK_STREAM)
         msg = sock.recv(1024)
@@ -575,6 +575,15 @@
     def _testFromFd(self):
         self.serv_conn.send(MSG)
 
+    def testDup(self):
+        # Testing dup()
+        sock = self.cli_conn.dup()
+        msg = sock.recv(1024)
+        self.assertEqual(msg, MSG)
+
+    def _testDup(self):
+        self.serv_conn.send(MSG)
+
     def testShutdown(self):
         # Testing shutdown()
         msg = self.cli_conn.recv(1024)
Index: Modules/socketmodule.c
===================================================================
--- Modules/socketmodule.c	(Revision 58966)
+++ Modules/socketmodule.c	(Arbeitskopie)
@@ -324,10 +324,25 @@
 #include "getnameinfo.c"
 #endif
 
-#if defined(MS_WINDOWS)
-/* seem to be a few differences in the API */
+#ifdef MS_WINDOWS
+/* On Windows a socket is really a handle not an fd */
+static SOCKET
+duplicate_socket(SOCKET handle)
+{
+	HANDLE newhandle;
+
+	if (!DuplicateHandle(GetCurrentProcess(), (HANDLE)handle,
+			     GetCurrentProcess(), &newhandle,
+			     0, FALSE, DUPLICATE_SAME_ACCESS))
+	{
+		WSASetLastError(WSAEBADF);
+		return INVALID_SOCKET;
+	}
+	return (SOCKET)newhandle;
+}
+#define dup(fd) duplicate_socket(fd)
 #define SOCKETCLOSE closesocket
-#define NO_DUP /* Actually it exists on NT 3.5, but what the heck... */
+#define NO_MAKEFILE /* socket handles can't be treated like file handles */
 #endif
 
 #ifdef MS_WIN32
@@ -344,6 +359,10 @@
 #define SOCKETCLOSE close
 #endif
 
+#ifdef NO_DUP
+#define NO_MAKEFILE
+#endif
+
 #if defined(HAVE_BLUETOOTH_H) || defined(HAVE_BLUETOOTH_BLUETOOTH_H)
 #define USE_BLUETOOTH 1
 #if defined(__FreeBSD__)
@@ -1978,8 +1997,13 @@
 	PyObject *sock;
 
 	newfd = dup(s->sock_fd);
-	if (newfd < 0)
+#ifdef MS_WINDOWS
+	if (newfd == INVALID_SOCKET) {
+#else
+	if (newfd < 0) {
+#endif
 		return s->errorhandler();
+	}
 	sock = (PyObject *) new_sockobject(newfd,
 					   s->sock_family,
 					   s->sock_type,