Why do I see duplicate packages when querying the rpm database?

When running 'rpm -qa' on the system, multiple copies of various RPMs are found. For example:

# rpm -qa | grep dbus-glib | sort
dbus-glib-0.22-12.EL.5
dbus-glib-0.22-12.EL.5

This is because the system has RPMs for different architectures installed. By using the --queryformat argument with rpm, this can be clearly seen:

#  ...