Remove duplicates

This is a slightly modified version of Steven Chamberlain's patch, found in #801161

It does nothing unless apt-setup/_DEVEL_0/repository has been preseeded. (see: apt-setup!11 (closed))

If that setting exists, then remove_duplicates() is called, which ensures that only the last instance of each package mentioned in the Packages file will be passed on for processing.

When used in conjunction with net-retriever!1 (merged), this allows one to have an additional repository in use during installation, from which to download alternative udeb versions.