Nowadays, any developer can create a packages for his application (and any of its dependencies) and publish it in self-hosted repositories. Users can easily add such repository to their system's sources list and bureaucracy problem's over. Well, some developers are doing this already - the only thing that keeps the rest of them is either ignorance or complexity of the packaging process.
I believe there's no need for an Android market clone (which is yet another centralized repository). What users may need is just a directory, pointing to external repos. Ubuntu market seems somehow promising (at least I remember seeing some dialogs like "you need to enable this source to install that package").
Content duplication is not a problem. A real problem is keeping the system up-to-date when you want to update some library, because of an important feature or bugfix. And it's nearly impossible with every application's bundling their own copy of that library, with some copies being actually incompatible forks (and a lot of copies being just different builds - think of different compiler versions - of exactly the same sources).