how to prevent data duplication in excel 97-03

Hi Mostwanted, and welcome to the forum.

You can do this using Data Validation, for example.

Select cells A1:A100 then click Data -> Validation.
In the DV window, on the Settings tab, select 'Custom' in the Allow drop-down box.
In the Formula box that then appears, type: =COUNTIF($A$1:$A$100,A1)=1 and click OK.

Now you shouldn't be able to type in duplicate entries. Hope that helps!