The problem seems to be reproducible easily:

megari@miyoko:/tmp$ mkdir duplicity-test
megari@miyoko:/tmp$ cd duplicity-test/
megari@miyoko:/tmp/duplicity-test$ ls
megari@miyoko:/tmp/duplicity-test$ mkdir cache
megari@miyoko:/tmp/duplicity-test$ mkdir a
megari@miyoko:/tmp/duplicity-test$ mkdir b
megari@miyoko:/tmp/duplicity-test$ cat > a/foo.txt
Hello!
megari@miyoko:/tmp/duplicity-test$ duplicity --archive-dir=/tmp/duplicity-test/cache --name test --no-encryption a file:///tmp/duplicity-test/b
Local and Remote metadata are synchronized, no sync needed.
Last full backup date: none
No signatures found, switching to full backup.
Warning, found incomplete backup sets, probably left from aborted session
--------------[ Backup Statistics ]--------------
StartTime 1319589284.00 (Wed Oct 26 03:34:43 2011)
EndTime 1319589284.02 (Wed Oct 26 03:34:44 2011)
ElapsedTime 0.03 (0.03 seconds)
SourceFiles 2
SourceFileSize 4103 (4.01 KB)
NewFiles 2
NewFileSize 4103 (4.01 KB)
DeletedFiles 0
ChangedFiles 0
ChangedFileSize 0 (0 bytes)
ChangedDeltaSize 0 (0 bytes)
DeltaEntries 2
RawDeltaSize 7 (7 bytes)
TotalDestinationSizeChange 150 (150 bytes)
Errors 0
-------------------------------------------------

megari@miyoko:/tmp/duplicity-test$ duplicity collection-status --archive-dir=/tmp/duplicity-test/cache --name test --no-encryption file:///tmp/duplicity-test/bLocal and Remote metadata are synchronized, no sync needed.
Warning, found incomplete backup sets, probably left from aborted session
Last full backup date: Wed Oct 26 03:34:43 2011
Collection Status
-----------------
Connecting with backend: LocalBackend
Archive dir: /tmp/duplicity-test/cache/test

Found 0 secondary backup chains.

Found primary backup chain with matching signature chain:
-------------------------
Chain start time: Wed Oct 26 03:34:43 2011
Chain end time: Wed Oct 26 03:34:43 2011
Number of contained backup sets: 1
Total number of contained volumes: 0
 Type of backup set:                            Time:      Num volumes:
-------------------------
Also found 0 backup sets not part of any chain,
and 1 incomplete backup set.
These may be deleted by running duplicity with the "cleanup" command.
megari@miyoko:/tmp/duplicity-test$ nano a/foo.txt 
megari@miyoko:/tmp/duplicity-test$ duplicity --archive-dir=/tmp/duplicity-test/cache --name test --no-encryption a file:///tmp/duplicity-test/b
Local and Remote metadata are synchronized, no sync needed.
Warning, found incomplete backup sets, probably left from aborted session
Last full backup date: Wed Oct 26 03:34:43 2011
--------------[ Backup Statistics ]--------------
StartTime 1319589353.91 (Wed Oct 26 03:35:53 2011)
EndTime 1319589353.92 (Wed Oct 26 03:35:53 2011)
ElapsedTime 0.00 (0.00 seconds)
SourceFiles 2
SourceFileSize 4116 (4.02 KB)
NewFiles 0
NewFileSize 0 (0 bytes)
DeletedFiles 0
ChangedFiles 1
ChangedFileSize 20 (20 bytes)
ChangedDeltaSize 0 (0 bytes)
DeltaEntries 1
RawDeltaSize 27 (27 bytes)
TotalDestinationSizeChange 128 (128 bytes)
Errors 0
-------------------------------------------------

megari@miyoko:/tmp/duplicity-test$ duplicity collection-status --archive-dir=/tmp/duplicity-test/cache --name test --no-encryption file:///tmp/duplicity-test/b
Local and Remote metadata are synchronized, no sync needed.
Warning, found incomplete backup sets, probably left from aborted session
Last full backup date: Wed Oct 26 03:34:43 2011
Collection Status
-----------------
Connecting with backend: LocalBackend
Archive dir: /tmp/duplicity-test/cache/test

Found 0 secondary backup chains.

Found primary backup chain with matching signature chain:
-------------------------
Chain start time: Wed Oct 26 03:34:43 2011
Chain end time: Wed Oct 26 03:35:53 2011
Number of contained backup sets: 2
Total number of contained volumes: 0
 Type of backup set:                            Time:      Num volumes:
         Incremental         Wed Oct 26 03:35:53 2011                 0
-------------------------
Also found 0 backup sets not part of any chain,
and 2 incomplete backup sets.
These may be deleted by running duplicity with the "cleanup" command.
megari@miyoko:/tmp/duplicity-test$ duplicity cleanup --archive-dir=/tmp/duplicity-test/cache --name test --no-encryption file:///tmp/duplicity-test/b
Local and Remote metadata are synchronized, no sync needed.
Warning, found incomplete backup sets, probably left from aborted session
Last full backup date: Wed Oct 26 03:34:43 2011
Found the following files to delete:
duplicity-inc.20111026T003443Z.to.20111026T003553Z.vol1.difftar.gz
duplicity-full.20111026T003443Z.vol1.difftar.gz
Run duplicity again with the --force option to actually delete.
megari@miyoko:/tmp/duplicity-test$ ls b/
duplicity-full.20111026T003443Z.manifest
duplicity-full.20111026T003443Z.vol1.difftar.gz
duplicity-full-signatures.20111026T003443Z.sigtar.gz
duplicity-inc.20111026T003443Z.to.20111026T003553Z.manifest
duplicity-inc.20111026T003443Z.to.20111026T003553Z.vol1.difftar.gz
duplicity-new-signatures.20111026T003443Z.to.20111026T003553Z.sigtar.gz
megari@miyoko:/tmp/duplicity-test$