Add way to confirm duplicates of files aren't different editions

[WhaleJ84]

As a lot of book bundles contain the same books, when hb_organiser comes across these, it skips them as one version has already been copied to the destination.

It has yet to be confirmed but there is a chance that a newer edition of the book would be named the same as the older edition. In this case, if the older edition was copied first, the newer will be ignored.

This could be circumvented by checking file-sizes and verifying the md5 files of both versions.