I did my post office run yesterday mailing out a few books.  This morning one of the people who I had sent a book to asked that I cancel the order if I had not already sent it.  Apparently she already received a copy.  When I initially accepted the request, I printed the wrapper out pretty quickly but I took a few days to mail it.  So there were at least a couple of days that I had known she did not need the book I could have canceled the request.  Now is it just out of my hands?  The book should still be marked received and everything right?

What probably happened is that she ordered the book and it went to someone else-who accepted, printed the label and did not mark it mailed.  Since you had already mailed your copy out and marked it mailed in time-she doesn't have to give the 1st person a credit.  It gives a warning that if you don't mark a book mailed in time that you might not get your credit.  So she should mark yours received.  You followed the rules.  It's up to her if she wants to be nice and give a credit for the 1st book or let them learn their lesson. 

If the member is honest, she should mark the book received when it arrives.  It's certainly not your fault that she ordered two of the same book.  Just let her know that the book is already in the mail. 

This happened to me. The first sender did not mark it mailed and it rolled to the next person who did mail it. I go them both on the same day!! The second person actually gets the credit. I offered the first sender 2 credits (audio book) and she just told me to keep it. I ended up sending her an audio book from my shelf so noone actually lost out.

It's unfortunate the timing was such that you couldn't cancel the transaction as a courtesy.  But you've done nothing wrong Katherine, and should receive your credit without problems.  About all you can do at this point, is polite note to let the receiver know your book was already gone before you received her pm.

I replied to the requestor letting her know that the book was already in the mail and some of the possible ways you have suggested the double books may have happened.  She hasn't responded yet, but worst case she can just relist the book, right?  It's a wishlist book and it only costs 2.07 to mail.