assertion error when restoring backup (lp:#1566372)

[Original report](https://bugs.launchpad.net/bugs/1566372) created by **Jeff (westie76)**
```
Ubuntu 14.04.4 LTS
deja-dup        30.0-0ubuntu4
duplicity       0.6.23-1ubuntu4.1
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
100, in add_filename
    (self.volume_name_dict, filename)
AssertionError: ({1: 'duplicity-full.20160402T104304Z.vol1.difftar.gz', 2:
'duplicity-full.20160402T104304Z.vol2.difftar.gz', 3: 'duplicity-
full.20160402T104304Z.vol3.difftar.gz', 4: 'duplicity-
full.20160402T104304Z.vol4.difftar.gz', 5: 'duplicity-
full.20160402T104304Z.vol5.difftar.gz', 6: 'duplicity-
full.20160402T104304Z.vol6.difftar.gz', 7: 'duplicity-
full.20160402T104304Z.vol7.difftar.gz', 8: 'duplicity-
full.20160402T104304Z.vol8.difftar.gz', 9: 'duplicity-
full.20160402T104304Z.vol9.difftar.gz', 10: 'duplicity-
full.20160402T104304Z.vol10.difftar.gz', 11: 'duplicity-
full.20160402T104304Z.vol11.difftar.gz', 12: 'duplicity-
full.20160402T104304Z.vol12.difftar.gz', 13: 'duplicity-
full.20160402T104304Z.vol13.difftar.gz', 14: 'duplicity-
full.20160402T104304Z.vol14.difftar.gz', 15: 'duplicity-
full.20160402T104304Z.vol15.difftar.gz', 16: 'duplicity-
full.20160402T104304Z.vol16.difftar.gz', 17: 'duplicity-
full.20160402T104304Z.vol17.difftar.gz', 18: 'duplicity-
full.20160402T104304Z.vol18.difftar.gz', 19: 'duplicity-
full.20160402T104304Z.vol19.difftar.gz', 20: 'duplicity-
full.20160402T104304Z.vol20.difftar.gz', 21: 'duplicity-
full.20160402T104304Z.vol21.difftar.gz', 22: 'duplicity-
full.20160402T104304Z.vol22.difftar.gz', 23: 'duplicity-
full.20160402T104304Z.vol23.difftar.gz', 24: 'duplicity-
full.20160402T104304Z.vol24.difftar.gz', 25: 'duplicity-
full.20160402T104304Z.vol25.difftar.gz', 26: 'duplicity-
full.20160402T104304Z.vol26.difftar.gz', 27: 'duplicity-
full.20160402T104304Z.vol27.difftar.gz', 28: 'duplicity-
full.20160402T104304Z.vol28.difftar.gz', 29: 'duplicity-
full.20160402T104304Z.vol29.difftar.gz', 30: 'duplicity-
full.20160402T104304Z.vol30.difftar.gz', 31: 'duplicity-
full.20160402T104304Z.vol31.difftar.gz', 32: 'duplicity-
full.20160402T104304Z.vol32.difftar.gz', 33: 'duplicity-
full.20160402T104304Z.vol33.difftar.gz', 34: 'duplicity-
full.20160402T104304Z.vol34.difftar.gz', 35: 'duplicity-
full.20160402T104304Z.vol35.difftar.gz', 36: 'duplicity-
full.20160402T104304Z.vol36.difftar.gz', 37: 'duplicity-
full.20160402T104304Z.vol37.difftar.gz', 38: 'duplicity-
full.20160402T104304Z.vol38.difftar.gz', 39: 'duplicity-
full.20160402T104304Z.vol39.difftar.gz', 40: 'duplicity-
full.20160402T104304Z.vol40.difftar.gz', 41: 'duplicity-
full.20160402T104304Z.vol41.difftar.gz', 42: 'duplicity-
full.20160402T104304Z.vol42.difftar.gz', 43: 'duplicity-
full.20160402T104304Z.vol43.difftar.gz', 44: 'duplicity-
full.20160402T104304Z.vol44.difftar.gz', 45: 'duplicity-
full.20160402T104304Z.vol45.difftar.gz', 46: 'duplicity-
full.20160402T104304Z.vol46.difftar.gz', 47: 'duplicity-
full.20160402T104304Z.vol47.difftar.gz', 48: 'duplicity-
full.20160402T104304Z.vol48.difftar.gz', 49: 'duplicity-
full.20160402T104304Z.vol49.difftar.gz', 50: 'duplicity-
full.20160402T104304Z.vol50.difftar.gz'}, 'duplicity-
full.20160402T104304Z.vol1.difftar')
```

Story points: 40.0