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this looks like the same issue as #430160.  and here's what 
I just wrote there...

it appears that IE is sending 2 extra bytes ('\r\n') after 
the request data.  and if you don't read those two extra 
bytes off, the window's socket handling gets messed up.

the result is that a partial response is returned and the 
socket closed.  IE tries to recover by re-POST'ing (which 
is behavior specified in the HTTP/1.1 RFC)... only they 
seem to add an embedded NULL the second time through, and 
the original socket problem happens again anyway.

Try reading an extra 2 bytes from the rfile before sending 
your response and the problem should go away.  (you can do 
that by 'self.rfile._rbufsize = content_length + 2' inside 
your do_POST method before reading the content)

not sure what the real fix for this should be?