I have the same question. Can someone please answer?
I'm having the same problem. Up to 5 copies of some books I've downloaded from the library. It's as if every time I open the program, it imports another instance of the books.
I had the same frustrating problem on a Macintosh laptop. Here's how I solved the problem.
First, look in your Applications folder for any alias of Adobe Digital Editions and delete that alias file, (leave the actual ADE application file alone, do not delete it). Then in the Finder, hold OPTION and select the Go pulldown menu. Library will appear when you hold the OPTION key down. Navigate to Library > Application Support > Adobe > Digital Editions. If there are any .dat files in that folder besides activation.dat then delete them. Empty your trash. Restart your computer. Open ADE and the duplicate files should be gone.
I had to delete a few duplicates. I chose the most recent dates and deleted those files. The files never returned.
On a PC you'll probably have to do something similar. Look for the folder containing activation.dat and do the above.