Ban duplicate keys in parameter files

[jamart28]

  root_level: DEBUG
  root_level: INFO

The above is currently allowed as YAML libraries handle duplicates differently, some allowing and some not. Instead this should be checked for and disallowed. Most likely duplicates will have to be checked manually something like the following: https://gist.github.com/pypt/94d747fe5180851196eb.

[jamart28]

Doing more research, there are plenty of workarounds, although I'm not sure where they should be implemented in our code since it involves changing the behavior of yaml.safe_load() (though that is no really an issue as a place can be found in the many workarounds). However, it looks like PyYAML has actually begun working to correct this divergence from the YAML standard: yaml/pyyaml#165. As such do we want to implement a workaround for now and take it out after their fix goes live or wait for their fix? Currently, the halt in work comes from keeping implementation of merging while disallowing duplicates. @gabbard @berquist

[gabbard]

@jamart28 : Let's hold off on this awaiting the upstream fix.