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Originally Posted by jony08

Please add a function to automatically delete one of the duplicates if it has a certain format compared to the other. For example, I want to delete all PDF files automatically if another format is available.

Originally Posted by Tanjamuse

Or any other columns? Word count is lowest or last-edited date?

I think this issue has been addressed more than once by kiwidude. It is outside the scope of this plugin to decide which books to keep and which to delete. He also noted several times that this is better implemented in a separate plugin.

And even if one is to write a separate plugin to handle this, there is a lot of difficulties in implementing it; most obvious is the question:

How to decide which book(s) to delete and which to keep? Every user has his own set of criteria which makes it difficult to write a plugin that satisfies the need of all users (short of writing a separate plugin or routine for each user).

So the best way is for each user to implement his own routine, by writing python scripts and running them through calibre-debug. This has the obvious problem that most users don't code and cannot go down this path. But even for people who can code, and want to write their own scripts to handle their unique individual needs this can be challenging:

• Let's take for example the first request of deleting books that have only pdf formats if other formats exists. A lot of time you find that you have two duplicate entries each containing a pdf and epub formats. Now you have to decide which one of them to delete. Do I delete one of them randomly? Or maybe I should implement another set of criteria for such occurrences.
• One solution to this is to keep the last edited, as suggested in the second request. But now I have another problem, a lot of times they will all have the same modification time because calibre resets the modification date in a lot of situations (for example, whenever you add a custom column calibre resets the modification date for all books in the library).

So now I have to decide on some additional criteria to determine which books to delete, which will lead me further down the rabbit hole, until I finally realize it actually easier to manually choose which books to delete from the GUI.

That being said if someone can and want to implement this feature, all power to them.