Hi
It generally doesnt do that. If it does it for a specific package you should report it as a bugNot sure if anyone here's commented on the AMD-64 experience. Both Fedora and Ubuntu are very good on AMD-64. But they have weaknesses in their x86 support.
On Fedora, if you parallel-install AMD-64 and x86 RPMs, then just remove the x86 one, in my experience RPM will remove the shared files too, which it probably shouldn't. (Which can be a problem if you decide to nuke x86 support and remove the glibc x86-arch RPMs: it breaks language support...)
regards Rahul