yes, I can run an analysis over a snapshot of our union catalog 
comprising of around 4.847.212 ISBNs. It will take some time to write 
the code.

Due to the official policy of the ISBN agencies, the reuse was strictly 
forbidden - but the ISBN number pool is segmented into countries and 
their publishers, which take charge over the number use. There is no 
technical mechanism to enforce correct use or to grant or revoke ISBNs 
by a third party. Beside applying the same ISBN to different editions, 
there are other cases. Some publishers wanted to save resources and 
simply broke the ISBN rules when they ran out of money (or they were 
rejected from receiving more ISBN numbers). They started to recycle 
ISBNs of books they had out of print for many years, hoping no one will 
ever notice. Or, publishers did not properly file their ISBN pool usage. 
For example, when publishers took over other publisher's business and 
their ISBN pools, there was no safe way of verifying what ISBNs were 
already taken or not. ISBN was in active use as primary identifiers in 
ISBN registers for no longer than 5 or 10 years. It's giving us 
headaches for a very long time. Do not rely on ISBN as a unique identifier.


Am 16.05.13 23:54, schrieb Ford, Kevin:
>   Is there any way to quantify, for example, how often publishers actually reuse ISBNs in different editions (is that even tecnically permitted?)?