pep8
Option to check for duplicates while copying between libraries

Add an option to check for duplicates (books with the same title/author)
when copying between libraries. Option is under Preferences->Adding
books. Fixes #1245089 [When copying or moving book to different library, Calibre does not warn if possible duplicate exists in target library.](https://bugs.launchpad.net/calibre/+bug/1245089)