Upgrading 9.0.5 to 9.0.13 on ubuntu 10.4

Hello Everybody,
I am going on an assumption that I am running into a bug, but before I log it, I wanted to see if perhaps anyone else has ran into it and maybe be able to shed some light on it.

os: Ubuntu 10.4
upgrading from PostgreSQL 9.0.5 to 9.0.13
using: apt get
with the following repository: http://apt.postgresql.org/pub/repos/apt/dists/lucid-pgdg/
doc: http://www.postgresql.org/download/linux/ubuntu/

Summary: trying to use apt-get to install Postgresql 9.0.13 on top of 9.0.0 results in an error and a state where the client package is installed and at 9.0.13 and the core package remains at 9.0.5. Trying to install all of the packages results in an error about plpython, following these attempts with a the force option on installing the core is successful.

Question: why do I have to force the installation of the core package?

apt-get install postgresql-client-9.0
- apt get detected that postgresql-9.0 is a dependency
Error:
The following packages have unmet dependencies:
postgresql-client-9.0: Breaks: postgresql-9.0 (< 9.0.13-1.pgdg10.4+1) but 9.0.5-1~lucid is to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

apt-get -f install postgresql-9.0 postgresql-client-9.0 postgresql-contrib-9.0 postgresql-plperl-9.0 postgresql-plpython-9.0 postgresql-pltcl-9.0 postgresql-server-dev-9.0
Error:
Preparing to replace postgresql-9.0 9.0.5-1~lucid (using .../postgresql-9.0_9.0.13-1.pgdg10.4+1_amd64.deb) ...
* Stopping PostgreSQL 9.0 database server [ OK ]
Unpacking replacement postgresql-9.0 ...
dpkg: error processing /var/cache/apt/archives/postgresql-9.0_9.0.13-1.pgdg10.4+1_amd64.deb (--unpack):
trying to overwrite '/usr/share/locale/zh_TW/LC_MESSAGES/plpython-9.0.mo', which is also in package postgresql-plpython-9.0 0:9.0.5-1~lucid
dpkg-deb: subprocess paste killed by signal (Broken pipe)
Errors were encountered while processing:
/var/cache/apt/archives/postgresql-9.0_9.0.13-1.pgdg10.4+1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

apt-get -f postgresql-9.0
- was successful

Thank you,
Kasia