If the buyer thought they had only been mounted once: ask the buyer how much he think he should be compensated for this difference, then go to the shop and demand that much of a difference(as long as its reasonable)
If the buyer knew it had been mounted twice: go to the shop and let them know, I would look for free tune ups for a year, or possibly a refund on the mount, something along those lines depending what you personally want to gain...
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Black Ladies WatchMen