Maybe I missed understood something but the browser will only send back last stored information if you don't clear of variables and what is on the screen between subsequent displays. I mean if the user enters the data for the GR and hits submit then if you create the GR clear all variables and also clear the variables that will fill the screen before sending a response back to the browser, any refreshes of the browser from this point onwards would not cause a duplicate GR because the last stored information the browser has will not generate a GR because you have cleared the variables you then displayed in you html.
If all that is correct, then the only problem comes from users getting impatient during GR posting and pressing refresh which would then resend duplicate information. If this is the only case causing problems have you tried using <xhtmlb:protectDoubleSubmit >? Although it only freezes the screen during process and not standard browsers buttons like refresh and back, it does show a very clear pop up box to let them no that the system is processing and then I would say it is the users fault if they press refresh in the mean time. You put this tag somewhere near the top of you main page or view and it will create a popup to freeze the screen while the browser is waiting for response e.g.
<xhtmlb:protectDoubleSubmit
title = "tile"
text = "Please Wait"
Then you could remove the logic to PO, quantity and delivery and just relay on the fact that the screen will get cleared before they press refresh.
