Same happens when download a file from a browser, and later download same file again. The [number in [brackets] indicates file already exists and saves like this when user has opted out to be asked what to do when file name exists. Since altbinz when adding a file normaly always has a filename that can be edited, maybe be easier if you change it there.
Don't think would be any other reason to change this, or maybe only for rss if is set to auto save files and auto download nzb. Even then if same nzb name is found may probably interact with first rather than second nzb first. (if rss does this). Or if par2 doesn't work with the files if cannot find the expected filenames to work with.