fix: Properly identify non-unique index in non-pk tables

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[tswast]

Let's add some unit tests specificaly for this method with index_cols = [] and primary_keys = []; index_cols = ['some_col']

[TrevorBergeron]

added some unit tests

[tswast]

Thanks!