Comment 9 for bug 845136

The double packages are not duplicates, the packages have different sizes.
Unfortunately, aptitude does not show any other attribute that differs, so I have no idea what exactly is happening there; it could well be a multiarch issue. I took a handful of samples and it seemed that script-only packages were there only once, but that could be an accident.

I disabled the multiarch file by renaming it to .multiarch.
I got a lot of updates, plus it wanted to remove flashplugin-installer and nspluginwrapper, which didn't come unexpected.

However, adobe-flashplugin, while installing cleanly, will not make Youtube work again.
Installing flashplugin-installer would fail because it wanted flashplugin-downloader and nspluginwrapper.
I had to go to Adobe's page, which in turn sent me to Canonical's "partner" repository. Installing now, hopefully it will work...