I mailed a book this morning, from PA to GA, and marked it mailed before lunch. Now at about 4 PM, I get a credit and an email that the book was received. Is this possible? Or could it be one of those multiple copies roaming the countryside sort of things I've read about here? I'm sort of waiting to see if there are any other developments, but I'm also not worrying about it. Everything looks ok. It just...looks a little odd, too, if you know what I mean.

Most likely, it's from a first sender that never marked the book as mailed. The transaction cancelled and went to you, the first book arrived, and the person marked the book received never knowing that there were two people involved. 

It shouldn't be a problem since, when this happens, it's the second sender that is supposed to get the credit anyway. It's not your (or the requester's) fault that the original sender never marked their book mailed. You might want to send the person a PM letting them know what's happened though.

Here's the info from the help docs

What Lisa said

Also, a member occassionally gets in a rush and marks the wrong book received. 

I once received an "instant" credit due to the other member's account being suspended.