Unable to add books - "the document that you are attempting to add already exists in the library" - there is NOTHING in the liabrary. Any suggestions??

I  just downloaded Digital Editions so I can read proceedings from a conference. I followed the instructions but cannot add the

proceedings to ADE. The error message reads "Unable to add document xxx.epub. The document that you are attempting to

add already exists in the library" There is nothing in the library, can't add the "Getting started" . e-book - same message. I did

delete ADE and re-downloaded it. Is there anything else I can do? Any help would be appreciated. Thanks