First, look at the DDL of the index on the target table to see the columns that are in the unique index idx1.
Then, run the following query on the source table (assuming col1, col2, col3 are the columns in the unique index) to see the duplicate rows.
count(*) > 1This will tell you the combinations of col1, col2, col3 that are not unique.
At this point, what you want to do depends on your business requirements. For example you may choose to insert only one row from each of the duplicate sets or you may choose to remove the unique constraint on the target table etc.