Backup fails with unknown error (assertion failure in duplicity)

Deja Dup and/or duplicity is failing for me now. This might have been triggered by my being away from the machine when the SSH passphrase prompt came up, and the session timing out. Or that may be a red herring.

The "Backup Failed / Failed with an unknown error:" window has this traceback in it:

Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1494, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1488, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1337, in main
    do_backup(action)
  File "/usr/bin/duplicity", line 1370, in do_backup
    globals.archive_dir).set_values()
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 697, in set_values
    self.get_backup_chains(partials + backend_filename_list)
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 819, in get_backup_chains
    map(add_to_sets, filename_list)
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 809, in add_to_sets
    if set.add_filename(filename):
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 96, in add_filename
    self.set_manifest(filename)
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 127, in set_manifest
    remote_filename)
AssertionError: ('duplicity-full.20160527T063251Z.manifest.gpg', 'duplicity-full.20160527T063251Z.manifest')

$ lsb_release -d
Description: Ubuntu 14.04.4 LTS
$ dpkg-query -W deja-dup duplicity
deja-dup 30.0-0ubuntu4
duplicity 0.6.23-1ubuntu4.1