6. Charles Robertson, owner of Quality Auto Parts, was preparing checks for payment of the current month’s purchase invoices when he realized that there were two invoices from Best Car Products Company, each for the purchase of 100 car batteries. Robertson thinks that Best Car Products Company must have billed Quality Auto Parts twice for the same shipment because he knows the shop would not have needed two orders for 100 car batteries within a month.

1. How can Robertson determine whether Best Car Products Company billed Quality Auto Parts in error or whether Quality Auto Parts placed two identical orders for 100 car batteries?

2. If two orders were placed, how can Robertson prevent duplicate purchases from happening in the future?