How are books added? Order

Hey,

I have a folder with 2 dozen subfolders that each have atleast 150-200 ebooks in them.
I had to abort adding the books around 60% because of a computer issue. But I can't figure out where the books stopped being added from. I've tried the first folder and last folder and those are all duplicates. Is there an oder calibre follows in adding books from multiple folder? I don't want to add all of them again. I know it goes over duplicates more quickly but it still can take awhile.

Thanks