Duh,
If you're a single, straight, male and your dream girl was, ahem, "ready" for you in your bedroom, what would you do?
If you're on a deserted island and had no food but a big wave washed up some sushi, I mean, live fish into a tidal pool, what would you do?
P.S. seriously, the mathematical probability of someone typing in the wrong payment address is so astronomically low, I think it must be some debt someone owed you from long ago that you forgot about or have already written off as a deadbeat. There's no way money just "accidentally" drops in your lap.
One time, I found a guy's wallet on the ground. I mailed the money and id back to him, but cut up and threw away the ratty old wallet and credit cards. It didn't make sense spending $4 of my money to mail back a bunch of credit cards that he probably already cancelled and a ratty old wallet that was worthless. It makes more sense to spend approx 40 cents to mail him back $20 of his money and his ID.
So, yeah, if they emailed me for the money, I'd return it. However, I wouldn't recommend blindly just sending the money back because the return address could be a temporary one. For example, if they sent the money using
https://shapeshift.io , they wouldn't get the money back if you sent it back to the sending address.