This is the first time this has happened to me so I'm not sure how to proceed. 

I requested a book FIFO from Member A, who accepted the request and printed the wrapper. But they never marked it as shipped and so the time expired. The request moved on to Member B, who accepted and printed the wrapper the same day and mailed it the next day. After B had already marked the book as mailed, A messages me saying they're so sorry they didn't get the book mailed, they got busy in real life with sick pets and other things, and did I still want the book. I messaged back saying I understood about real life difficulties and I was sorry it happened to them but that another member had already gone through with my order.

Today, the day after I responded to A's message and B has already mailed the book, A responds back saying that their son accidentally mailed the book yesterday, thinking he was doing a favor for Member A.

So now I'm going to end up with two copies of the same book. Do I have to give Member A a credit when their book arrives, even though their transaction timed out? 

In this situation (the first sender didn't mark the book mailed in time AND you got the second book), you aren't required to mark the first book (the one that timed out) as received. You should mark the second book (the book that was marked mailed) received.

You will not be required to use 2 credits since you only submitted 1 request. You can read how to handle the situation in the Help Center document *I received 2 copies of the same book!*

Erin--Something similar happened to me on the very first book I received from PBS.  I e-mailed the sender who didn't mark the book mailed and asked her if she would be willing to order a book directly from my bookshelf.  That way, I could send her book back to her along with the book she'd ordered from me.  She accepted. It probably cost me a few cents more in postage to mail two books rather than just the one, but I got a credit to cover the shipment and she got her original book back, plus hopefully a new to her book that she was interested in reading.

You have the option to mark it received in your Transaction Archive.  But you do not have to. 

(Please note that everyone should be VERY careful when they go to their TA.  Both canceled and shipped orders are there.  It is how we mark "Lost" books received.  But anytime we mark a book there, a credit is deducted.  And there is no way to undo it.  There is a lot of great info there.  But be careful.)

Actually, I've thought of a friend who I can give the book to. So I'll give Member A the credit, since the book will be put to good use. But thanks for all your help.