I found a workaround for the Duplicity fails to start issue where
'sudo deja-dup' would fail with the python stacktrace mentioned in the launchpad ticket.
The ticket was not very useful, so I started looking at the various files in the stacktrace and saw line from /usr/lib/python3/dist-packages/duplicity/backends/giobackend.py was within an
"if u'DBUS_SESSION_BUS_ADDRESS' not in os.environ" block.
So I wondered what would happen if I let that environment variable pass into the sudo environment. I tried
'sudo -E deja-dup' as per preserve the environment. This didn't result in a stacktrace, but it ended up running the backup as the normal non-root user, probably because the preserved environment included the USER and HOME variables along with the DBUS_SESSION_BUS_ADDRESS variable.
Then I tried preserving just DBUS_SESSION_BUS_ADDRESS with
'sudo --preserve-env=DBUS_SESSION_BUS_ADDRESS deja-dup', it worked as expected.
So the hint here is that when presented with a stacktrace don't be afraid to "Use the Source, Jean Luc".