Problem with library after ADE transfer to new computer

I have installed ADE on a new computer and authorised it, but when I try to move epub documents from the 'My Digital Editions' folder into the library I get a message saying 'The document you are attempting to add already exists in the library' and yet I can't see any documents there. I have tried uninstalling and installing the older 2.0 version, but this hasn't helped.