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On Windows, I froze a script that uses multiprocessing with cx-freeze and Python 3.4.1 that had freeze_support() in the "if __name__ == '__main__'" section of the main module, and the resulting executable crashes with a RuntimeError 'context has already been set'.
The error happens in a call to set_start_method in multiprocessing.spawn's prepare function, and changing the line in spawn.py from:
if 'start_method' in data:
 set_start_method(data['start_method'])
to:
if 'start_method' in data:
 set_start_method(data['start_method'], force = True)
allows the frozen executable to successfully run.