WNforce Uniqueness on Calculated column

Hi TEam,

 I have a sharepoint list where I created a column by combining two other columns, and now i want to enforce unique values on that column. How to do it?

Third column=[Number]&[EmailID]

number can take multiple values, email id can take multiple same emails but both feilds together shouldnt be same in 3 rd feild.

PLease help me on this