added check for already existing option names to OptionGroup.addoption()

[userzimmermann]

_pytest.config.OptionGroup.addoption() didn't care about adding the same options twice. Only argparse library was later complaining about the doubled options. That made it quite difficult to find the actual root cause of issue #1618, because you couldn't get any information from the exception traceback why the doubled option adding happened. Now you can track down such issues very easily.

[coveralls]

Coverage increased (+0.003%) to 92.358% when pulling bb3ef0e on userzimmermann:sprint/addoption-check into a2420ce on pytest-dev:features.

[coveralls]

Coverage decreased (-0.009%) to 92.389% when pulling 69bab4a on userzimmermann:sprint/addoption-check into 7d60fcc on pytest-dev:features.