With the option Desktop and Documents active in iCloud Drive settings on an iMac all Digital Editions purchases are copied to the iCloud Drive because they are stored in the local ~/Documents/Digital Editions folder which, as I learned, cannot be changed.
The Digital Editions app on the iPhone holds another copy of all purchases in local storage which are also backed up to iCloud.
As a result, all purchased Digital Editions books require double storage space both on the iPhone and in iCloud: one copy for the Digital Editions app and another copy on the iCloud Drive folder /Documents/Digital Editions.
Storage consumption costs money and energy.
How can I resolve this unnecessary duplication?
Please refrain from obvious suggestions like deactivating the iCloud Drive option Desktop and Documents.