This is a recipe rehashed from https://gitlab.com/duplicity/duplicity to avoid the "librsync.h missing" error.
First install pipx with:
python3 -m pip install --upgrade pip pipx
Followed with:
pipx ensurepath
Then install librsync with macports:
sudo port install librsync
And finally:
export CPATH=${CPATH}:/opt/local/include; export LIBRARY_PATH=${LIBRARY_PATH}:/opt/local/lib; pipx install duplicity