Don't you have access to the SAP Marketplace yourself to search for notes? service.sap.com/notes is the address
1090663 is a workaround in without GR based invoice verification
103051 explains the design with GR based invoice verification and a workaround in case of inter company
So the only gap that I see is an invoice from a third party having several items referencing the same PO item.
You did not answer the question: What is the reason that you have the same item referenced multiple times in GR based invoice verification?