[duplicityBackup] Check duplicity's lockfile.lock

[C-Duv]

duplicity uses a file-based lock mechanism to avoid running simultaneous backups of the same source.
It creates a file named lockfile.lock in the archive directory (--archive-dir), in a subdirectory named as per the symbolic name of the backup (--name) or the hash the backend URL.
Example of location: ~/.cache/duplicity/my-foo-bar-backup/lockfile.lock.

If the duplicityBackup is run multiple time simultaneously, duplicity will correctly detect the presence of a lockfile and won't process extraneous backups: so everything is OK.
But I think script should check if duplicity's lock file exists before trying to run duplicity: it would avoid unnecessary processing and fail faster.