Fatal error: KeyError: 1 while restoring incremental backup (lp:#751178)

[Original report](https://bugs.launchpad.net/bugs/751178) created by **ChieftainY2k (chieftainy2k)**
```
I've just tried the restore procedure, but it seems that it fails under
ubuntu with the latest version of duplicity installed (from tar.gz):

root@chieftainy2k:~# uname -a
Linux chieftainy2k 2.6.35-28-generic #49-Ubuntu SMP Tue Mar 1 14:39:03 UTC
2011 x86_64 GNU/Linux

root@chieftainy2k:~# python -V
Python 2.6.6

root@chieftainy2k:~# duplicity -V
duplicity 0.6.13

root@chieftainy2k:/tmp/restore# duplicity -v9 restore
ftp://USER@SERVER/backup/DIR ./
........ (cut) ...........
Collection Status
-----------------
Connecting with backend: FTPBackend
Archive dir:
/home/chieftainy2k/.cache/duplicity/3d571ef397a9af4d38eaa5356d94eacd

Found 0 secondary backup chains.

Found primary backup chain with matching signature chain:
-------------------------
Chain start time: Sat Mar  5 13:58:46 2011
Chain end time: Wed Mar 30 17:21:39 2011
Number of contained backup sets: 17
Total number of contained volumes: 320
 Type of backup set:                            Time:      Num volumes:
                Full         Sat Mar  5 13:58:46 2011               234
         Incremental         Mon Mar  7 15:49:41 2011                 7
         Incremental         Mon Mar  7 18:39:47 2011                 5
         Incremental         Tue Mar  8 18:09:43 2011                 2
         Incremental         Wed Mar  9 11:55:35 2011                11
         Incremental         Wed Mar  9 18:02:20 2011                 2
         Incremental         Fri Mar 11 12:25:15 2011                16
         Incremental         Mon Mar 14 10:42:37 2011                12
         Incremental         Tue Mar 15 17:51:25 2011                 4
         Incremental         Fri Mar 18 17:06:37 2011                 5
         Incremental         Mon Mar 21 17:01:32 2011                 5
         Incremental         Wed Mar 23 16:47:17 2011                 4
         Incremental         Thu Mar 24 17:00:35 2011                 2
         Incremental         Sat Mar 26 17:34:12 2011                 2
         Incremental         Sun Mar 27 17:07:15 2011                 2
         Incremental         Mon Mar 28 19:05:42 2011                 3
         Incremental         Wed Mar 30 17:21:39 2011                 4
-------------------------
No orphaned or incomplete backup sets found.
Removing still remembered temporary file /tmp/duplicity-gil1cb-
tempdir/mkstemp-vb6llm-1
Removing still remembered temporary file /tmp/duplicity-gil1cb-
tempdir/mkstemp-ycLH7u-2

Traceback (most recent call last):
  File "/usr/local/bin/duplicity", line 1250, in <module>
    with_tempdir(main)
  File "/usr/local/bin/duplicity", line 1243, in with_tempdir
    fn()
  File "/usr/local/bin/duplicity", line 1197, in main
    restore(col_stats)
  File "/usr/local/bin/duplicity", line 539, in restore
    restore_get_patched_rop_iter(col_stats)):
  File "/usr/local/lib/python2.6/dist-packages/duplicity/patchdir.py", line
521, in Write_ROPaths
    for ropath in rop_iter:
  File "/usr/local/lib/python2.6/dist-packages/duplicity/patchdir.py", line
493, in integrate_patch_iters
    for patch_seq in collated:
  File "/usr/local/lib/python2.6/dist-packages/duplicity/patchdir.py", line
378, in yield_tuples
    setrorps( overflow, elems )
  File "/usr/local/lib/python2.6/dist-packages/duplicity/patchdir.py", line
367, in setrorps
    elems[i] = iter_list[i].next()
  File "/usr/local/lib/python2.6/dist-packages/duplicity/patchdir.py", line
112, in difftar2path_iter
    tarinfo_list = [tar_iter.next()]
  File "/usr/local/lib/python2.6/dist-packages/duplicity/patchdir.py", line
328, in next
    self.set_tarfile()
  File "/usr/local/lib/python2.6/dist-packages/duplicity/patchdir.py", line
322, in set_tarfile
    self.current_fp = self.fileobj_iter.next()
  File "/usr/local/bin/duplicity", line 575, in get_fileobj_iter
    backup_set.volume_name_dict[vol_num],
KeyError: 1


What am I doing wrong ?
```

Story points: 6.0