Raise on duplicate keys

[jakirkham]

Recently we had an issue in cuCIM where the same key appeared twice ( rapidsai/cucim#757 ). Unfortunately we didn't realize until something didn't behave correctly

It would be helpful to catch these cases in advance by having RDFG raise if it sees duplicate keys

[jakirkham]

cc @grlee77 (who caught this issue)

[jameslamb]

Just ran into another cases of this: rapidsai/rmm#1613 (comment)

Alternatively, we could roll out something like yamllint across RAPIDS. It has a rule to prevent duplicate keys: https://yamllint.readthedocs.io/en/stable/rules.html#module-yamllint.rules.key_duplicates

That could probably be used for some of the other things I mentioned in #103 as well.