While I can't help with your situtation, I can tell you that when we visited China my wife's visa was voided and caused some issues at the airport. They let us get boarding passes, and we didn't understand the issue (in our case it was a pen mark across it). We looked through the passport trying to understand if it was an accidential mark or what, and saw another visa in the back. They spelled her name wrong on the first one (though it was only a missing space, nothing we would care about).

The point was, the airline would have been perfectly happy to let us fly with that, indeed they mentioned it to us that was a common way to void it, we had just thought it was a pen in the purse or something. I totally agree with you, I would not have been happy if they turned us around if the other vaild one wasn't in there, and as soon as they saw it marked, they immediatly looked for another one.

Did you use a tour company or visa agency for the visa? They should have noticed it and gotten it corrected (as they did with ours, they got the passports back and then sent hers out again), though not sure you will get anything going that route.