pyyaml should disable duplicate keys

[zsfbct]

The YAML spec disables duplicate keys. When there exist duplicate keys, pyyaml doesn't catch it as an error but simply override.

For reference: https://stackoverflow.com/questions/53920358/pyyaml-loading-yaml-1-1-with-duplicate-keys

It should check keys uniqueness with similar logic to the code below in constructor.py

        if key in mapping:
             warnings.warn(somewarning)
        else:
            mapping[key] = value

[perlpunk]

duplicate of #165
Especially see my comment #165 (comment)
So fixing it is not that trivial, but PRs that keep existing merge key behaviour are welcome.