Are the ISBNs of two copies of a book the same?
As noted in the comments, the purpose of an ISBN is to uniquely identify a set of books, all of which contain precisely the same content.  Thus, it is reasonable to expect that two books with precisely the same ISBN will have precisely the same content.

It is worth noting, however, that the reverse does not apply.  Two books may have precisely the same content but be issued different ISBNs because they come from different printings.  Likewise, printing errors, etc., can in rare cases render two books different despite having the same ISBN. For nearly all cases, however, the one-to-one relationship holds.