Duplicity gives a warning when run that an import of httplib2 failed. This should probably be added as an RDEPEND.
I'm assuming you were trying to use the Ubuntu One backend? Duplicity has a lot of optional backends each having their own runtime deps that would be better placed in some sort of suggested/optional deps format.
Actually, it gives the warning regardless of backend used, and I have no need for Ubuntu One. Might be worth an einfo or something. I'd be on the fence about that, actually - agree that suggested deps would be better if we had them.
I've added a post install message about optional deps. If you want the warning messages to go away you should take that request upstream.