Accidentally sent a huge invoice to a nightmare client for work I didn't do. They paid it, and I'm keeping the money

About a year ago, I had a corporate client that was easily the worst working experience of my life. Extremely rude, treated me like shit, constantly calling me during non-work hours. They always demanded I do more outside the scope of the contract. I eventually managed to miraculously meet all their *requirements* and finished the job, even though it went 2 months over schedule (which I never got paid for) So a few months ago, I flew back home after completing a project for a new client. I was totally sleep deprived but just wanted to get things wrapped up. Typed up the typical thank you email, attached the final invoice, hit send, and crashed for the night. Woke up the next day and checked my phone. Saw a notification from my bank about a deposit, and another auto-email I get when an invoice is marked paid. Then I saw a third email from the *new* client I just finished with. I opened that, and it basically said *Hey, loved the work, just send over the invoice when you can so we can get you paid.* I was so confused while reading through it. I thought I literally just sent them the invoice last night??? I didn't think I had any other outstanding payments. I closed that email and went back to check the payment confirmation. That’s when it hit me. The payer name was the disastrous corporate client I worked with last year. I must have autofilled the wrong email address in my sleep deprived state, as the first three letters are the same. But regardless, their accounts payable must be such a disorganized mess that they didn't even check if the invoice was valid, they just paid it. It was a large sum as well. That should tell you a lot about just how disastrous this client is. After thinking it through, I decided to use that to my advantage, because fuck them. I took the money and set it aside into a safe investment account, just in case they ask for it back. But at this point it's been months, and I haven't heard anything from them. I plan on keeping the money in the account for a year. But I am not going to be proactive in giving it back. Considering all the unpaid overtime from last year, I'm calling us even. tldr; Accidentally sent a large invoice to an abusive client I haven't worked with in a year instead of my new client. They paid it without checking. I'm keeping the money as back pay for the hell and unpaid overtime they put me through.