Seeing as this post is about dogs, I thought I'd weigh in.

If multiple mailings really are being sent to everyone, then you have a legitimate gripe.

But if it's only an occasional slip-up where someone accidentally gets sent two cards every once in a while, I don't see it as a big deal. Investing in the quality control system to prevent all mistakes would cost more than the total cost of the occasional slip ups that each cost 45 cents in postage and maybe a dollar in making up the card and getting it sent out.