"First, a Web proxy will not recognize that two copies of a package retrieved from different URLs are
identical, undermining the process for RPM-based distributions like Fedora, where the Yum update
tool incorporates built-in mirroring."
This is not really true. Squid can be setup to normalize URLs, so that the same package will be found
when two different URLs are used. I used to maintain a transparent proxy using that trick to handle
my colleagues using different Debian mirrors.