It's easy to introduce duplicate rows of data into Oracle tables by running a data load twice without the primary key or unique indexes created or enabled.
Here's how you remove the duplicate rows before the primary key or unique indexes can be created:
DELETE FROM our_table
WHERE rowid not in
GROUP BY column1, column2, column3... ;
Here column1, column2, column3 constitute the identifying key for each record.
Be sure to replace our_table
with the table name from which you want to remove the duplicate rows. The GROUP BY is used on the columns that make the primary key for the table. This script deletes each row in the group after the first row.