[kovidgoyal]

When you use Save to Disk on two (or more books) that would result in exactly the same file path (because your save to disk template, is, for example, {rating}/{title} and they have the same rating and title, then instead of overwriting one with the other, calibre will add a number in brackets to the end.

That number will increase with the number of such conflicting files, it should not be adding multiple brackets, however, that sounds like a bug. I will look at it when I have a moment.

Note that exactly the same file path means everything upto the file extension.