Finding double-billed shipments before the carrier's invoice ages out

Carrier invoices contain duplicates. Not many, and not usually in bad faith, but enough that any operation shipping across several carriers and several services will find some on any given month. A parcel gets relabeled and both movements get billed. A return leg is invoiced as a forward leg. A charge appears in this cycle and again in the next one because the first was disputed and re-presented.