How to make a custom column unique in sharepoint?

in sharepoint i created workflow, and in tasks lists i added 3 custom columns

my requirement is in those 3 , 1 column should be unique, For example, i added columns, empId,name dept,

in those empId should be unique, if any one who enter the same empId while creating, it won't accept,

is it possible?

any ideas, thanks in advance