I graduated with a PhD after 2 years (I was very efficient) and even though I have already received my degree and moved out of town, I'm still getting a stipend deposited into my bank account. I contacted my advisor about it but my advisor didn't reply at all, so I don't know what to do about this. Is there a minimum funding requirement or something like that? Or is it just a mistake? What to do now?
Contact your department, CC'ing your advisor.
It happened to me once: I was paid over the summer by my department even if I was out of the US for an internship at a company research lab in Europe. I came back in September and found additional money in my US account. I told our administrator. she figured out what happened and she told me that in case I had already spent the money, the department would have found a way to ensure things would get straighten out with the university. I still had all of it, so I just wrote a check to the university to return the money.
This is like someone giving you wrong change or the bank crediting you money incorrectly in your checking account. You need to not spend it, let someone know and give the money back. Two reasons: (1) ethical--don't take money that doesn't belong to you, when given in error and (2) they will likely find out about it and want it back (and legally you have to give back a check or direct deposit sent in error).
Don't contact your professor, contact the payroll or whatever person like that. Make a call and send a letter.
P.s. If your current job overpays you in error, how are you going to handle that? What would you answer if this hypothetical was asked on a job interview?! You know the answer. [This isn't even really an academia question. This is a "what do I do if someone pays me too much in error" question.]
Is there a minimum funding requirement or something like that?
This happened in my case. There was a total amount of money dedicated to a project, payed in the form of a stipend. I finished the project earlier but the money continued to come.
Of course you should contact the relevant authorities (the ones who provided the stipend) and inform them about your case. This may be an error, in which case you are likely to be asked to give the money back (what happens depends on many parameters), or you may be in the lucky case I was in.