I realized that an update of MiaB would probably reinstall duplicity on top of our snap symlink. So you’ll also want to block duplicity from being installed via apt. We do this by creating an apt preference file:
$ sudo echo -e "# Duplicity is installed via snap\nPackage: duplicity\nPin: release *\nPin-Priority: -1" > /etc/apt/preferences.d/duplicity
Now if you try to install duplicity it can’t:
user@mailbox:~$ sudo apt install duplicity
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Package duplicity is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'duplicity' has no installation candidate