Comment 2 for bug 876081

This is very bizarre. You have both a duplicity-full.20110923T204745Z.vol34.difftar.gpg and duplicity-full.20110923T204745Z.vol34.difftar.gz in the backup location.

I don't know why you have an encrypted volume 34 as well as an unencrypted one. If you move duplicity-full.20110923T204745Z.vol34.difftar.gpg out of the way and try again, do things work like you expect?