Experiment Validator should catch duplicates in lists

[thcrock]

We recently hit an error where the list of percentiles in evaluation config had a duplicate. We could of course squash duplicates in Triage, but I think this would be subpar as it means an incorrect version of the list gets hashed. We should make users fix the config instead.

[thcrock]

Another reason to throw an error is the possibility that the duplication is the result of a desired value being missing.

Also, +1 on this given today's error (cc @ecsalomon )