Silent error when specifying duplicate feature group prefixes

[adunmore]

When specifying multiple feature groups with the same prefix, triage silently overwrites all of the resulting tables with the final table to be generated. This only returns an error after all features have been generated, when triage tries to query now nonexisting feature columns.

It looks like duplicate prefixes should be caught by validate.FeatureGroupDefinitionValidator._validate_prefixes. However, the list of prefixes gets silently deduplicated by a set operation in defaults.fill_feature_group_definition.