`Config.known_args_namespace` contains duplicate value

[bluetech]

While looking at another issue I noticed that pytest -Werror results in config.known_args_namespace.pythonwarnings having the value ['error', 'error']. That is the value is duplicated.

This happens because:

• We call the arg parser twice
• -W is defined as append

It's not actively harmful for -W but might be for other cases. Possibly it only affects builtin options so might only affect -W -- I didn't dig too deep.

The second parse was added in 0a27f69