Duplicate keys should cause an error

[mhauru]

The YAML spec says that a mapping is "an unordered association of unique keys to values". ocaml-yaml seems to accept duplicate keys and just picks the first value:

utop # Yaml.of_string "{dada: 0, dada: 1}" |> Result.get_ok |>  Yaml.Util.find_exn "dada";;
- : Yaml.value option = Some (`Float 0.)

I would propose checking for duplicate keys and treating them as invalid YAML.