R12 Payables: Invoice numbers can be duplicated

Even though this main article is directed at R12.2, I know we have had this issue reported in our R12.1.3 instance AND the linked document in MOS article "R12:AP:Duplicate Invoice Numbers Got Created For Same Supplier and Site" from ID 2094877.1 is for all versions of R12 after R12.0.1 so consider yourself warned that a custom index needs to be created to prevent this from happening to you!