Duplicity incremental backup archives much bigger than necessary (lp:#1125225)

[Original report](https://bugs.launchpad.net/bugs/1125225) created by **gcc (chris+ubuntu-qwirx)**
```
Duplicity on our mail server eats 3 GB of our 6 GB /var partition for its
"cache".

Every day, the incremental backup creates a new ~200 MB signatures file,
such as duplicity-new-
signatures.20130211T040207Z.to.20130212T040221Z.sigtar.gz.

This file contains (large) signatures of not just the files that have
changed, but a random selection of files which have NOT changed. For
example, of our current set, the following files contain ~15 kB signatures
of a file that has not changed since 2004:

chris@one-mail(foo)$ for i in
/var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.*.sigtar.gz; do sudo tar tzvf $i
signature/home/sytse/horde-2.2.5.tar.gz && echo "yes: $i" || echo "no:
$i"; done
[sudo] password for chris:
no:  /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130120T040216Z.to.20130121T040206Z.sigtar.gz
no:  /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130121T040206Z.to.20130122T040206Z.sigtar.gz
no:  /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130122T040206Z.to.20130123T040214Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130123T040214Z.to.20130124T040206Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130124T040206Z.to.20130125T040211Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130125T040211Z.to.20130126T040205Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130126T040205Z.to.20130127T040212Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130127T040212Z.to.20130128T040210Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130128T040210Z.to.20130129T040212Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130129T040212Z.to.20130130T040208Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130130T040208Z.to.20130131T040221Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130131T040221Z.to.20130201T040208Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130201T040208Z.to.20130202T040209Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130202T040209Z.to.20130203T040210Z.sigtar.gz
no:  /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130203T040210Z.to.20130204T040205Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130204T040205Z.to.20130205T040209Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130205T040209Z.to.20130206T040207Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130206T040207Z.to.20130207T040210Z.sigtar.gz
no:  /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130207T040210Z.to.20130208T040210Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130208T040210Z.to.20130209T040219Z.sigtar.gz
no:  /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130209T040219Z.to.20130210T040211Z.sigtar.gz
no:  /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130210T040211Z.to.20130211T040207Z.sigtar.gz
no:  /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130211T040207Z.to.20130212T040221Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130212T040221Z.to.20130213T040226Z.sigtar.gz
yes: /var/cache/duplicity/fb4efb5e7e414ab421bd5fa2f90954de/duplicity-new-
signatures.20130213T040226Z.to.20130214T040220Z.sigtar.gz

Can anyone think why Duplicity might be storing this redundant information?
As the incremental backups are fairly useless without the full backup on
which they depend, I don't think redundant storage of unchanged signatures
is necessary or helpful.

Cheers, Chris.
```

Story points: 6.0