As my friend David says, good ideas are a dime a dozen. Well David, here's another one I had, then found. I hate when that happens.
“Data de-duplication, also called data de-dupe, removes duplicate information as data is backed up or archived. It can be done on the file level, where duplicate files are replaced with a marker pointing to one copy of the file, or at the sub-file level, or byte level, where duplicate bytes of data are removed, resulting in a decrease in storage capacity requirements of several magnitudes.”
OK, so the idea isn't unique, but if someone wants to bring that tech the desktop and you're interested in my UI ideas for it, I've got plenty.