I also had a book that was mailed late and I was on the WL. 

My book was marked mailed on 10/20, but  has a postage metered stamp of 10/31.  I was on the WL, so when it went lost I immediately went to the top of the list.

The day after it went missing, my request was accepted and printed, and my lost book showed up in my mailbox. 

There is nowhere on my end to cancel the request for the second book.  I did send a PM to the second person asking him or her to please not mail it until we get this all sorted out, but I don't know how to do that.

Am I correct that the second person will lose the top FIFO slot if s/he cancels the transaction on that end? 

How do I keep the second person, and myself, from being penalized for what the first sender screwed up?

Please help!

Once the sender has printed the wrapper, only the sender can cancel, so hopefully he'll see your PM and will cancel the transaction.  Then he can post the book again, and it will go to the next person on the wishlist.  Since it is a wish list book, he is not in line, so he won't lose his place.

If worse comes to worse, and he mails the book, you can post it when you receive it.

Here are some help docs:

I received a book that was 'lost in the mail'

I received 2 copies of the same book!

If the sender does cancel the transaction, you will go back on the wish list. Make sure to delete yourself from there. If the sender is having trouble, ask him to use the Contact Us link at the bottom of the screen. I'm pretty sure the PBS team can cancel books even if they've been marked mailed.