Even if it's rare, it means a database like goodreads can't assume that an IBAN is linked to only a single book.

It's rarer than cases of movie titles being reused, so in the context of the article, it's more than adequate, since letterboxd's title-based movie lookup is stated to be a better system.

For example, here's a non-exhaustive but still pretty long list of movies with reused titles: https://www.imdb.com/list/ls083468410/