I'm running Oneiric 64-bit, and since I've upgraded I notice Synaptic has split the repositories into 64-bit and 32-bit, but it's downloading archive lists for both of them, taking twice the time as before. Can anyone explain why this is and whether I can stop it?
oneiric now has support for multi-architecture packages. So, even though you are using a 64-bit system, you can now install 32-bit packages on it. Earlier, this was not possible. Currently, this support is also available in Debian Wheezy, which will be released sometime in 2012-2013.
I do not remember the exact name this multi-architecture support over the top of my head, but when I do, I'll update this post.
Also, I don't remember seeing any mention of how to disable this feature.
It's not specific to synaptic either. I see duplicated packages in the list with aptitude (lol I just tried to tab-autocomplete that!) but not with [the new] muon in Kubuntu 11.10 (oneiric) which appears to hide this. I assume they're all using apt behind the scenes or some library that is compiling the package list.
My upgrade didn't go well, this caused me problems; it seems that aptitude is not set up to handle this.
I was able to fix it like this :
Now aptitude only sees the native architecture packages.
Again this is not a specific answer but it fixes the problem you present IMO.