[Hampshire] Open source network backup with de-dupe.

Hi,

I have used a fair amount of different open source network backup
products, e.g. rdiff, bacula etc.
None of them do de-dupe in order to reduce the amount of data sent over the WAN.
Does anyone know of any open source backup programs that do de-dupe
for the express purposes of reducing traffic over the WAN.
I know de-dupe programs tend to use HASHs to find matches. What
methods do they use to detect HASH collisions?
The main area of interest is the initial backup seeding, where a full
backup has to be taken which then is followed by incremental ones.
If two sites have the same data, and one site has already sent the
initial backup seeding to the central site, the second site should not
need to also send the same data.

Kind Regards

James