Job fired me and forgot to take me off payroll. Do I send them back the money?

Edit: alright alright friends thanks for the advice and opinions. All my friends have been telling me block them and keep the money so I needed new perspectives. I'll send them the check this weekend.

Long story short I was working for a small company who decided after 3 weeks I "wasn't a good fit," but was professional and did nothing wrong just didn't fit in. Whatever. and they'd pay me for the two weeks after my date of no longer being their employee.

So, I received my final direct deposit and all was well. Two weeks later (biweekly pay period) I noticed a credit to my bank account for the amount of my usual pay. I alerted them right away, and they asked me to send back the money in a check which I thought just added to their unprofessionalism.

It's been over a month since they asked me that and just now they're asking me to "advise status of payment." However I'm kind of annoyed that I had to go out of my way to remind them to take me off payroll after they randomly fired me for NO reason, and that despite my honesty they asked for it back. Also, aren't I going to be taxed on that income? And then I have to pay for stamps to be able to mail them the check? Should I ignore their texts and calls and keep the money, or send it back?