Steps I did:
1. Import a Amazon-Ebook
2. Convert it to EPUB
After Importing a book, Calibre seams to add IDs of books which are already in my library to new added book.
For Example:
"At Road's End (Pre-Aztec Trilogy, Prequel)" See attachment.
IDs: isbn:9781449016487, mobi-asin:B006SNV7O6
The MOBI-ASIN is correct, but the ISBN links to "The Secrets of Hearty Turkish Home Cooking (Volume 1)" by Murat Yegul. This book is also in my library. I cant believe that that amazon or the author put a wrong ISBN to the metadata. Also the wrong IDs are always refering to books I did add at the same time to Calibre.
When adding multiple Ebooks, Calibre seams to confuse the IDs of the books...