Find and Delete Duplicate Rows in a Table

To find duplicate rows:

SELECT column1, column2
  FROM table_name
 GROUP BY column1, column2
HAVING COUNT(*) > 1;

To delete duplicate rows:

DELETE FROM table_name a
 WHERE ROWID > (SELECT MIN(ROWID)
                  FROM table_name b
                 WHERE a.column1 = b.column1
                   AND a.column2 = b.column2
               );