Firstly I don't add hundreds of books as a group - at most two dozen a day.
If I identify that Book A and Book B are in fact the same book, I Mark them and then Show Marked books. I first decide which of the two has the 'better' metadata, that will be the one I keep - let's assume it's Book B. If Book A has a 'better' cover I open its book folder and drag/drop cover.jpg to the Book Details panel of Book B. I do the same with the book format files. If Book A had a format that I didn't have for Book B (that I wanted to keep) it too would be drag/drop copied to Book B.
Obviously 'better' is a subjective judgement - e.g. cover A might be a higher quality image than cover B, but if I find its design to be awful, I would probably keep cover B - unless it was similarly awful
I then delete the unwanted book (Book A). My backup process will archive the deleted book and replaced covers and formats, the process purges archived items greater than 90 days old.
I add to an Intake library, ensure the metadata basics are correct (authors, title) and add a '!newbook' tag. I then use the
Find Duplicates plugin's
Find library duplicates feature to check for duplicates in the target library. Because I am dealing in small numbers I deal with duplicates by not
moving the 'duplicate' in my Intake library to the Target library. I don't use the Automerge or the tags to apply feature in Add Books. I deal with any books that get left in the in the Intake library after moving the non duplicates much the same as I deal with in-library duplicates, and then I delete them.
My target libraries have VL for books tagged with '!newbook', once I've done whatever needs to be done for a book I remove the '!newbook' tag.
Regarding tagging books with the source. I have started to use the
GetFileName to assist in this, I get it to record the full path from which I can usually determine the source.
BR