duplicity-remover is a simple script written in Python that keeps N full backups and removes all backups that are older than that. You can use it to clean up duplicity backup files without having the PGP secret key.
Why do I not use the built-in function of duplicity? I download the encrypted backup to a Raspberry PI and duplicity remove-all-but-n-full requires the PGP secret key. For security reasons I don't want to have the key on the Raspberry so that's why I wrote this script.
Testet on Python 2.7.11
##Example
./duplicity-remover.py --remove-all-but-n-full 3 --force /backups/files
will keep 3 full backups and removes all older full and incremental backups. The '--force' option will delete the files. To try this script, you can leave it out.