This seems fairly unlikely to ever occur. The database would have to be decentralized, and the two items would have to be mailed from separate locations and both be scanned more or less simultaneously—or within however many minutes it takes for database changes to propagate, which can’t be all that long since you’re able to buy a code and use it on the same day.

Given how remote the probability of this happening is, simply ignoring it and delivering both pieces probably isn’t unreasonable. Or flag it for further investigation and maybe bill the sender or recipient for an additional piece of mail.