you need to create a recordset to see if the data being entered already exists. I would expect you wish to check for the first name last name combo, set the recordset to filter the first name, last name columns on the respective form elements.
next add sserver validation, use Number validation, set the variable to:
where <Recordset Name>is the name of your recordset.
set the minimum number to -1 and the Maximum to 0
This will cause validation to pass only if the recordset is empty.