I like when a compilation has no warning, because it helps to notice useful warnings and real bugs. The compilation of Python 3.5 on Windows emits the following warnings on buildbots:

xxmodule.obj : warning LNK4197: export 'PyInit_xx' specified multiple times; using first specification
xxmodule.obj : warning LNK4197: export 'PyInit_xx' specified multiple times; using first specification
xxmodule.obj : warning LNK4197: export 'PyInit_xx' specified multiple times; using first specification

http://buildbot.python.org/all/builders/AMD64%20Windows8%203.5/builds/1120/steps/test/logs/warnings%20%283%29

http://buildbot.python.org/all/builders/AMD64%20Windows8%203.x/builds/2634/steps/test/logs/warnings%20%283%29

It seems like the warnings come from test_distutils:
-----------------
0:05:09 [ 44/405] test_distutils failed (env changed) (38 sec)
xxmodule.c
xxmodule.obj : warning LNK4197: export 'PyInit_xx' specified multiple times; using first specification
   Creating library D:\Temp\tmp_crvgs9y\Debug\Temp\tmp_crvgs9y\xx_d.cp37-win_amd64.lib and object D:\Temp\tmp_crvgs9y\Debug\Temp\tmp_crvgs9y\xx_d.cp37-win_amd64.exp
LINK : /LTCG specified but no code generation required; remove /LTCG from the link command line to improve linker performance
foo.c
   Creating library D:\Temp\tmpnq__9cbm\tempt\Temp\tmpfv6uys06\foo_d.cp37-win_amd64.lib and object D:\Temp\tmpnq__9cbm\tempt\Temp\tmpfv6uys06\foo_d.cp37-win_amd64.exp
LINK : /LTCG specified but no code generation required; remove /LTCG from the link command line to improve linker performance
foo.c
   Creating library D:\Temp\tmpnq__9cbm\tempt\Temp\tmpfv6uys06\foo_d.cp37-win_amd64.lib and object D:\Temp\tmpnq__9cbm\tempt\Temp\tmpfv6uys06\foo_d.cp37-win_amd64.exp
LINK : /LTCG specified but no code generation required; remove /LTCG from the link command line to improve linker performance
xxmodule.c
xxmodule.obj : warning LNK4197: export 'PyInit_xx' specified multiple times; using first specification
   Creating library D:\Temp\tmp8n1b1bh8\Debug\Temp\tmp8n1b1bh8\xx_d.cp37-win_amd64.lib and object D:\Temp\tmp8n1b1bh8\Debug\Temp\tmp8n1b1bh8\xx_d.cp37-win_amd64.exp
LINK : /LTCG specified but no code generation required; remove /LTCG from the link command line to improve linker performance
foo.c
   Creating library D:\Temp\tmpk47tprte\tempt\Temp\tmpe3sksr_y\foo_d.cp37-win_amd64.lib and object D:\Temp\tmpk47tprte\tempt\Temp\tmpe3sksr_y\foo_d.cp37-win_amd64.exp
LINK : /LTCG specified but no code generation required; remove /LTCG from the link command line to improve linker performance
foo.c
   Creating library D:\Temp\tmpk47tprte\tempt\Temp\tmpe3sksr_y\foo_d.cp37-win_amd64.lib and object D:\Temp\tmpk47tprte\tempt\Temp\tmpe3sksr_y\foo_d.cp37-win_amd64.exp
LINK : /LTCG specified but no code generation required; remove /LTCG from the link command line to improve linker performance
xxmodule.c
xxmodule.obj : warning LNK4197: export 'PyInit_xx' specified multiple times; using first specification
   Creating library build\temp.win-amd64-3.7-pydebug\Debug\xx_d.cp37-win_amd64.lib and object build\temp.win-amd64-3.7-pydebug\Debug\xx_d.cp37-win_amd64.exp
LINK : /LTCG specified but no code generation required; remove /LTCG from the link command line to improve linker performance

D:\buildarea\3.x.bolen-windows8\build\build\test_python_3476>exit 1 

D:\buildarea\3.x.bolen-windows8\build\build\test_python_3476>exit 0
-----------------

By the way, test_distutils failed because of "env changed" but the changed environment is not logged :-( See my related issue #27829 for the missing warning.

This is a known bug when compiling 64-bit extension modules.

http://bugs.python.org/issue9709
http://bugs.python.org/issue16779
http://bugs.python.org/issue20166

> This is a known bug when compiling 64-bit extension modules.

Oh ok. I will just close my issue in that case :-)

I know I closed #20166 myself, but could we revisit this?

I'm now in favor of telling people to fix their modules.