The message at the following link to the Duplicity Update Broke Backups discussion which I posted using my individual account identifies the missing package as python-gnupg. The command lines that you quoted are from a portion of the the Updating System Software section of the Mail-in-a-Box Maintenance Guide that I updated earlier. Running those command lines in sequence should update your MiaB instance properly. Only if those command lines fail to produce a properly functioning instance should any package downgrade be considered. The root cause of the Duplicity 3.2 update problem has been fixed, so updating Duplicity normally should work reliably.