Comment 3 for bug 822697

I confirm this bug.

However I found a way of workaround it.

$ duplicity --verbosity 9 --exclude-other-filesystems --exclude-device-files --encrypt-sign-key 123664B6 --rsync-options "--bwlimit=500 --rsh=\"ssh -oBatchMode=yes -p 22 -i "/etc/cron.duplicity/duplicity_key"\" /home rsync://user@host:22//backup/home

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duplicity 0.6.18 (February 29, 2012)
Args: /usr/bin/duplicity --verbosity 9 --exclude-other-filesystems --exclude-device-files --encrypt-sign-key 123664B6 --rsync-options --bwlimit=500 --rsh="ssh -oBatchMode=yes -p 22 -i /etc/cron.duplicity/duplicity_key" /home rsync://user@host:22//backup/home
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Calling duplicity with --rsync-options and passing --rsh or -e as rsync option you can overwrite the default "-e" and include inside the call the path to the key.