Values Already Exists in the table

Question

I have one table like this ...
Pk     EmployeeiD             Date             Time
1     Emp1001  5/8/2012  00:03:02
2   Emp1001        5/9/201200:03:02
3   Emp1001   5/10/201200:00:49

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

If i 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)....

I didnt use Constraints in my table... I didnt know about constraints...am a Beginner