Using backupninja with duplicity on my servers since a while, I am very happy with both!
I use it with sftp/scp as backend (I don't care which, as both work, or rather a combination of them).

Encrypted incremental backups with an easy ncurses management frontend, that is what geeks like me like :)

then again, duplicity did change things, now it is broken on wheezy for sftp/scp:

Import of duplicity.backends.giobackend Failed: No module named gio
BackendException: ssh connection to backuphost.domain:22 failed

The gio error is false alarm, the other one is related to some duplicity internals.

After some fiddling I just decided to try the latest upstream version:
as deb ..

duplicity 0.6.21-0ubuntu1 deb on launchpad

or newer.

and, voila, works. again.
don't bother to fix this bug, just upgrade!!