Same ISBN, but different edition?

[Chrissi]

Hi, I recently bought the book with the ISBN 9780375832994 on Amazon (https://www.amazon.de/dp/0375832998?p...), where it is advertised as a "20th Anniversary Edition".

It does, however, have the same ISBN as the standard edition of the book, the one that the search returns when searching for the ISBN and that I attached to this topic.

As there is additional content at the end in the edition that I bought that the standard edition doesn't have, for me the question arises how I should handle adding this book.

Is it just an alternative cover edition or is it a new edition? And, if it is the latter of that, should I just leave the ISBN blank? I already looked through all the editions of the book and none of them have this this cover (one comes pretty close but has a sticker that states it as a "Barnes & Noble Exclusive", which, as a side note, makes its use highly questionable with regard to its source in my eyes).

Sorry, if the procedure may be obvious here, but I really am unsure.

Thanks in advance for help!