This is a suggestion from https://www.digitalocean.com/community/tutorials/how-to-use-duplicity-with-gpg-to-securely-automate-backups-on-ubuntu?comment=12940
asymmetrically encrypt the backup to a different public key by passing a different key id to the "--encrypt-key" flag"
Or cache the password using gpg-agent but there is a caveat that it will not survive a reboot.