how to see and delete duplicate rows .

select mgr,count(*) from emp
group by mgr
having count(*) >1;

MGR	COUNT(*)
7839	3
7698	5
7566	2

DELETE FROM your_table
WHERE rowid not in
(SELECT MIN(rowid)
FROM your_table
GROUP BY column1, column2, column3);

example-
select * from emp
where rowid not in (select min(rowid) from emp group by mgr);

EMPNO	ENAME	JOB	MGR	HIREDATE	SAL	COMM	DEPTNO
7844	TURNER	SALESMAN	7698	08-09-1981	1500	0	30
7900	JAMES	CLERK	7698	03-12-1981	950		30
7654	MARTIN	SALESMAN	7698	28-09-1981	1250	1400	30
7698	BLAKE	MANAGER	7839	01-05-1981	2850		30
7902	FORD	ANALYST	7566	03-12-1981	3000		20
7521	WARD	SALESMAN	7698	22-02-1981	1250	500	30
7782	CLARK	MANAGER	7839	09-06-1981	2450		10