collection-status removes local running backup files (sigtar)

Duplicity: 0.6.18-3 (debian stock)
Python: 2.7.3-4+deb7 (debian stock)
OS: Debian GNU/Linux 7.8 (wheezy)

We run duplicity --collection-status as part of periodic system health cron job (to feed passive nagios checks). The health script runs every 5 minutes.

During the normal backup run, on the local disk, signatures are written to $filename.sigtar.part. That file is then copied to $filename.sigtar and gzipped.

If $filename.sigtar.part is being copied to $filename.sigtar during a collection-status, duplicity will remove $filename.sigtar. These cause local backups to show as failed/incomplete, and for duplicity to fail with an error -- usually something along the lines of:

"OSError: [Errno 2] No such file or directory: '/var/cache/duplicity/$hostname/@/duplicity-full-signatures.$date-time-stamp.sigtar'"

This can be repeated, perhaps, by unzipping $filename.sigtar.gz and running collection-status. It can definitely be repeated by cp $filename.sigtar.part to $filename.sigtar, and running collection-status.