We need to update our backup duplicity to at least 0.6.26.
It contains a fix to a corruption bug on resuming that existed around 0.6.22 .
It also contains a lock file support now, which prevents concurrent runs on the same backend url.
Actually the newest version is 0.7 but it is under development :
Latest available release is still 0.6.22.
We should wait the new rpm is out.