parallel_bilby_generation should fail on duplicate settings in .ini

We've run into a problem a few times where (due to our own error) we had a duplicate setting in the .ini file ("duplicate" in the sense that the key was used twice, but actually different values were given; prior files in this case) and parallel_bilby_generation silently used one of them (I think the second).

It would be good to save the user from mistakes like this by making parallel_bilby_generation fail when encountering the same key twice in the .ini.