Here's what I do in the form's BeforeUpdate event:
(ID is an Autonumber primary key of my People table)
If Nz(Dlookup("ID","tblPeople","FirstName = '" & Me.tbxFirstName _
& "' AND LastName = '" & Me.tbxLastName & "'"),0) = 0 then
MsgBox "Cannot save record with duplicate name",,"Save Cancelled"
Cancel = -1
Exit Sub
End If
HTH,
Jim