This is why I'd prefer to use K4PC solely to download books into calibre. No need for background sync, just let K4PC download all new books by default (primary device when purchasing), all books will be added to calibre and deleted, they won't be redownloaded so no duplicates.
Find Duplicates PI can help you clean up.
Alternatively, leave the title alone and use a custom column to
write the name of the title, use that going forward.
But that is a really awkward workaround for keeping old copies of everything, ever, in your auto-add directory. Also, calibre has to read and import all those files, then delete them again!
WHY????