I'm backing up 2 drives to an external backup drive. The 2 source drives have a lot of duplicate files (name/size) between them (for work). One of the source drives is already backed up to the destination drive. I'd like to backup the second source drive to the same destination drive, different directories, but not include any files with the same name and size if they already exist somewhere on the destination drive. Is there a backup or copy program that can index the destination drive and then check the index for each source drive file, to prevent duplicate backed-up files scattered on the destination drive?