IMHO Calibre is warning you that a title EXISTS (and you might be doing something unwanted).
FWIW I get this with an Empty book.
Preferences Adding Books: Adding Actions has a Auto-merge setting where you say what to do if: xyz

I paste the Log into Notepad, then I can scroll and resolve issues.


Use an Intake Library (not a VL): The use the
Find Library Duplicates (target Library set)



I also can LOOK at the target using the Content server while the Intake is Active (does help if you have Muilt-monitors ar a second internet browsing device)