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Article:
  SQL Data Types
Subject:   Prove that the combination of two columns are unique
Date:   2006-08-18 07:52:04
From:   PatTheDBA
Response to: Prove that the combination of two columns are unique

Try this:

select column1 || column2, count(1)
from some_table
group by column1 || column2
having count(1) > 1


This will return all rows that are not unique and the number of duplicates