Prevent Duplicate in Msql

taken from here How to prevent Duplicate records from my table Insert ignore does not work here

alter the table by adding UNIQUE constraint

ALTER TABLE employee ADD CONSTRAINT emp_unique UNIQUE (ename,dno,mgr,sal)

but you can do this if the table employee is empty.
or if records existed, try adding IGNORE

ALTER IGNORE TABLE employee ADD CONSTRAINT emp_unique UNIQUE (ename,dno,mgr,sal)

UPDATE 1
Something went wrong, I guess. You only need to add unique constraint on column ename since eno will always be unique due to AUTO_INCREMENT.
In order to add unique constraint, you need to do some cleanups on your table.
The queries below delete some duplicate records, and alters table by adding unique constraint on column ename.

DELETE a
FROM Employee a
     LEFT JOIN
     (
        SELECT ename, MIN(eno) minEno
        FROM Employee
        GROUP BY ename
     ) b ON a.eno = b.minEno
WHERE b.minEno IS NULL;

ALTER TABLE employee ADD CONSTRAINT emp_unique UNIQUE (ename);

Here's a full demonstration

• SQLFiddle Demo