Get the last item added to RMS POS transaction Screen

I am creating an RMS Plugin to set the quantity of the last item added by the cashier through add item. The plugin works perfect but fails when the cashier enter a wrong itemlookupcode, this causes the plugin to change the quantity of a previously added item. My question, How do i know that the item was added successfully so that i can update the quantity since additem hook is called even when the cashier enters none existing itemlookupcode.
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