Since I upgraded to Oneiric apt-get seems to ignore my architecture and gets the sources for both, amd64 AND i386, files from the repositoreis. this leads to constant "duplicate source entries" errors.
What to do?
**EDIT: Information provided by dizzy
Here is the requested file (since I'm not allowed to post more than two URLs I've modified the URLs not to be recognized (to std url / http:// to ..):
deb *std url* oneiric main restricted
deb *std url* oneiric-updates main restricted
deb *std url* oneiric universe
deb *std url* oneiric-updates universe
deb *std url* oneiric multiverse
deb *std url* oneiric-updates multiverse
deb ..archive.canonical.com/ubuntu oneiric partner
deb *std url* oneiric-security main restricted
deb *std url* oneiric-security universe
deb *std url* oneiric-security multiverse
deb ..download.virtualbox.org/virtualbox/debian oneiric contrib # disabled on upgrade to oneiric
deb ..extras.ubuntu.com/ubuntu oneiric main #Third party developers repository
They enabled multiarch support in Oneiric, which breaks aptitude, see this bug entry on launchpad: https://bugs.launchpad.net/ubuntu/+source/aptitude/+bug/831768