I recieved in the mail a book that had been canceled by sender postmarked 09/30/2010. I have already recieved the book from another sender.I went into my transactions & the sender has never marked this mailed.

Do i have to give a credit ? I no longer need this book & was unaware that it was even mailed. I have had no pms from the sender asking about this book.

no you do not have to mark it received since it was cancelled by the sender (but most likely they didn't mark it mailed so it was cancelled by the system)

No need to mark it received or give credit to first Sender, since that transaction cancelled as not marked mailed, and since you have received another copy since then.  (You would only be obligated to mark the first book as received from your Transaction Archive if you hadn't received another copy from another transaction.)

No Pam, you do not need to mark the book received to send a credit.   From the HELP documents:

I received a book from a canceled transaction!

This usually happens when the member that sent this book did not complete the sending process by marking the book mailed, but sent the book anyway.  In these situations, there is a possibility that the requestor will end up with 2 copies of the book.

If you received (or will receive) two copies of the book, you are not obligated to give a credit to the sender whose transaction was canceled.  If you got (and will get) only one copy, you should mark the book received so that the sender gets credit.

ETA - Basically the same guideline applies for 'lost' books.