"Backups" (deja-dup): after installing duplicity 0.8 from source, "Backups" can't find it

To solve your original issue of Unicode errors, I would recommend trying the snap version of Deja Dup. It has the 0.8 version of duplicity bundled in.

If Deja-Dup does check for duplicity package instead of command, you may try faking a Debian package like in this case MySQL 5.6 depends on client 5.5 , which is about a dependency issue.

equivs may help to create dummy packages.

1. Install equivs:

   sudo apt-get install equivs
   

2. Generate control file from template:

   equivs-control duplicity
   

3. Open duplicity file for edit. Change name and version if needed:

   ...
   Package: duplicity
   Version: 0.8.05
   ...
   

4. Build then install it:

   equivs-build duplicity
   sudo dpkg -i duplicity*.deb
   

Notes:

• It can be uninstalled like any package. If you are using this method on large scale (company computers), write something in description (step 3) to indicate it's a dummy/fake package.
• For general case, some packages have Provides:.. field, check original package and copy that line from its control file, add it with step (3).

Programs can run with different PATH values. The most common that has bitten me is that sudo commands use a limited set of path values.

To Check the environment that a program is running with run ps axeww | grep deja-dup. The output will include the value of PATH used, towards the end of the output.

If the executable is not in the PATH used by deja-dup, you can add a symbolic link. For example to add a symbolic link in /usr/bin/ run a command similar to the following.

sudo ln -s /usr/local/bin/duplicity /usr/bin

Install snap version of duplicity, purge apt duplicity, and create link from snap to apt default location(/usr/bin/duplicity). And reboot.

See here: https://bugs.launchpad.net/ubuntu/+source/duplicity/+bug/1770929/comments/30