Comment by WorldMaker

ISBN reuse is a quite large problem for some especially small publishers. The intended use of an ISBN is a product code for a retail point of service. If the book prices were expected to be basically the same, reusing the same ISBN for an entire shelf of books was sometimes fine if the retailers didn't mind the inventory management problem of knowing which specific book titles were left. For "pulp paperbacks" intended for a spindle at a grocery store they probably didn't care to manage the inventory by exact title, they managed it by the spindle-full.