I don't know if only sending the changes is considered "De-Duping" (though it is related) - That's more equivalent to doing differential (or block level differential) backups.
De-Duping is if you add the same file (or portions of a file) in 10 different directories - the data is only stored once in backups.
Dropbox takes it a step further - if 1,000 people store the xcode DMG on their dropbox store, only one copy is stored in Dropbox.
I don't know if only sending the changes is considered "De-Duping" (though it is related) - That's more equivalent to doing differential (or block level differential) backups.
The way Tarsnap does things, it's the same thing; Tarsnap splits data into blocks and deduplicates those blocks without caring whether each block is a whole file or part of a file or several small files stuck together.
De-Duping is if you add the same file (or portions of a file) in 10 different directories - the data is only stored once in backups.
Dropbox takes it a step further - if 1,000 people store the xcode DMG on their dropbox store, only one copy is stored in Dropbox.