Values Already Exists

Question

I have one table like this ...
Pk     EmployeeiD      Date        Time

1	Emp1001	5/8/2012	00:03:02
2	Emp1001	5/9/2012	00:03:02
3	Emp1001	5/10/2012	00:00:49

the above values are inserted successfully.....in their respective dates.... 

If a insert the above values again in the same table  means

(ie..)Employeeid        Date            Time
        Emp1001        5/9/2012       00:03:02

it will show some message like (Date Already Exists)....