Relevant article:
What do they do with all the other copies they get?
I think they only get one copy of each title/edition/ISBN number unless that first copy is defective and can’t be scanned. Remember there are tens of millions of those published.
More likely they just scan them all and let God the Computer sort it out. At the scale they’re working at, it’s probably more trouble than it’s worth to have any sort of process for ensuring lack of duplicates.