John,
   In Form Design mode, select the field you want to monitor for dupes (ex.
MyField).
   In the properties box for that field, find the AfterUpdate event.
   Enter "Event Procedure" in the AfterUpdate text field. (no quotes)
       (OR... Using the down arrow to the right of the textbox, select
[EventProcedure])
   Click the  little three dot button that appears on the right of the
AfterUpdate text box. (...)
   You are now in the form's module, and you should see...

Private Sub MyField_AfterUpdate()

End Sub

Add your Refresh...

Private Sub MyField_AfterUpdate()
   Refresh
End Sub

Now, each time MyField's value changes, if it violates the NoDuplicates
rule, it should bark that right away.