calibredb export feature request

[McStubb]

when using the calibredb export command is it possible to include a "No Clobber" or exclude existing ebook files? My thoughts is cases where you start an export from a remote library, but for some reason you have to stop and re-start it, everything gets downloaded again. Would save time/bandwidth if calibre db would check to see if a matching ebook format already exists at the target location before downloading the book.