The accountant in me is freaking out. I used to be on a payroll team out of college and did some help with corporate AP stuff. It's honestly a little baffling. There should be records that US agencies have. That when they receive a bill. They can confirm and balance out hours, then pay it out. So that when a duplicate bill, even if there is a new invoice number, should cast serious doubt against their own records. Requiring confirming with the employee, even going up thru higher checks from a manager or rep (for a example, a sales rep). That should have been caught.