Flyblue, I don't think the query related to attempted multiple check-ins, but rather whether it is possible to have multiple boardings on a single online check-in.
It's not what I meant. As lexxity has explained better than I did, as Mr X has checked in/boarded, his status on the system will show as checked in/boarded, and your code rejected.
An attempt to re-check in/re-board by a second person would be immediately spotted. In case of 2 different people showing up with same boarding pass and same name, I think the Credit Card used to pay the ticket could be a proof.