When I add a new book, it can happen that a new author entry is created, even if an author with the same name exists. The name strings (first name, family name) are 100% equal.

When I store the book and reopen it, I can add the second author with the same name to the author list…

…and when I store the book, the duplicate entry seems to be merged – i.e., only one entry remains – so this is an existing handy way to resolve this behavior.

I am not sure if I can make this reproducible, but I have experienced it 3, 4 times in the past.