The documentation of the method "makefile" for socket
objects still claims that the new file object
corresponds to a dup()ped file descriptor, which seems
to be wrong.

Python Version: 2.3.2 
Operating system: Debian GNU/Linux

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Why does this seem to be wrong? 
If I look at the code in socketmodule.c, it seems to me that
sock_makefile does a dup() on the socket's fd, and returns a
file based on the new fd....

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I can no longer understand what made me file this bug
report. I vaguely remember having had the problem that no
dup was done,
but I am no longer sure what I have seen at the time.
Sorry for the inconvenience!

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I'm pretty sure I know why you filed this item.  socket.py
contains code to work around shortcomings on Windows and BeOS 
(neither were able to dup() sockets).  At some point we
incorporated that
same code for other platforms for consistency reasons, thus
underlying
sockets are not really dup'd:

>>> import socket
>>> s = socket.socket()
>>> s
<socket._socketobject object at 0x81d9dec>
>>> t = s.dup()
>>> t
<socket._socketobject object at 0x81df02c>
>>> t._sock
<socket object, fd=3, family=2, type=2, protocol=0>
>>> s._sock
<socket object, fd=3, family=2, type=2, protocol=0>
>>> id(s._sock)
136563280
>>> id(t._sock)
136563280

Most of the time this doesn't affect anything, but I believe
it does
in certain cases.