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The root cause of having the flags twice is that the Makefile explicitly sets LDFLAGS in the environment before when running setup.py, and that distutils appens the value of $(LDFLAGS) to $(LDSHARED) when LDFLAGS is set in the environment.
Line from the makefile:
 $(RUNSHARED) CC='$(CC)' LDSHARED='$(BLDSHARED)' LDFLAGS='$(LDFLAGS)' OPT='$(OPT)' ./$(BUILDPYTHON) -E $(srcdir)/setup.py build;
And in Lib/distutils/ccompiler.py (in customize_compiler):
 if 'LDFLAGS' in os.environ:
 ldshared = ldshared + ' ' + os.environ['LDFLAGS']
A quick workaround to remove the duplicate flags is to explictly remove the related values from os.environ before calling main in setup.py:
# --install-platlib
if __name__ == '__main__':
 import os
 del os.environ['LDFLAGS']
 main()
I've attached a patch that implements this behavior. I'm not 100% sure this is the right solution because sysconfig._parse_makefile also looks at the environment and it may be better to remove the code from ccompiler.customize_compiler instead.