Whose fault is this (work related question)

An external data form has to be sent out each month by a required deadline. Colleague A completes it and colleague B reviews it. It’s interchangeable as to who submits it to the external company. Colleague B has been off work for a few days, when they return there is no correspondence that colleague A has submitted it. Therefore colleague B submits it today to meet the deadline, as colleague A is now off for a week. It turns out colleague A had already sent it but hadn’t informed colleague B. So the company has now received it twice. Who would you say is at fault?