How guilty is he?

Consider a ..hypothetical case.

Due to a certain fuck up, a large national corporation used to pay P's electricity bill. Being a good honest individual he reported their mistake to their office, and offered them a payment equal to the amount they paid. They said they would contact him again.

The following month they paid his electricity bill again. He reported this incident again to them, while checking whether the person he talked to have the authority to act on the corporation behalf. This time, they did send someone to him. He paid them although he only received their proof of payment as receipt, which wasn't enough evidence that the money would be properly received by the corporation.

..as you can guess, they paid his bill again on the following month. He called them again. They say they would get to him again. He waited and waited for months but no one ever come anymore. Meanwhile, he was getting so busy he totally forgot. When he finally remembered again, he didn't bother to contact that corporation anymore.

Interestingly, he later noticed that whenever he paid the bill earlier, they won't double pay for that month. That means they had a mechanism to check that a bill they were suppose to pay, was paid. That should raise a red flag on their system. He guess someone probably took advantage of that..

How guilty do you think is he?