Multilple libraries

[GeeDub]

I have several Calibre libraries from other computers, some with only one file type and others with multiple file types; epub, mobi, pdf. There are duplicates between them. I have had varying results in using the add books from the toolbar - sometimes there have been books left in a few of the folders without being added. (I noticed this that the some books appeared without format because JPEG was brought into Calibre I noticed this when I tried to read the book then use the properties to find it was a JPEG) It seems that Calibre wants to bring in other than book file types, which makes me wonder which folder type to use with the “add book’ feature in the toolbar. Does the JPEG and OPF files count as the same book but different format? I wonder if you can drag-and-drop the entire folder and have Calibre determine how to add the books.
I have had no success in merging two libraries and ensuring that the duplicates are not added. I wonder if it is better to use the ‘add book’ function for the whole folder, instead of the ‘Copy to’ feature. It does seem that the ‘Copy to’ functions allow for two formats to be merged which is a nice, though I am not sure why you would want when you can just convert later.
I find it odd that when making a copy of the library within Calibre takes several times longer than making a copy of the same folder outside of Calibre and then locating that folder within Calibre under create library.
Sorry this is long-winded, but I have been trying to figure this out by myself without too much luck.
Thanks!

[BetterRed]

I use the Find Library Duplicates feature of the Find Duplicates plug in to remove the books in the 'incoming' library that duplicate books in my 'main' library. I use the Copy to Library feature to merge the de-duped 'incoming' library into my 'main' library

This works better for me than the Add Books duplicate handling.

I use Add Books if I'm fairly certain that what I'm adding does not duplicate what's already in my main library.

BR