Do not want something repeated in a certain column

[Guest]

I have a column that will be filled with screen names (testing new software) and in this column I do not want the users to enter the same screen name twice. How do I keep them from doing it??? Thanks.

 

[anno]

use data validation. a method of preventing duplicate entries is at http://support.microsoft.com/default.aspx?scid=kb;en-us;Q189005