A CD-ROM form sent twice (a double click, a resent request) added a
second drive with the same ISO. The form is refused when the ISO is in a
CD-ROM of the VM's running or persistent definition; an empty drive may
still be added more than once. tr and zh too.
libvirt takes the same disk file or MAC twice in a persistent
definition. A double click or a resent request then attached an
existing volume twice, and a shut-off VM did not start again (QEMU
opens no file twice for writing), or added a second NIC with the MAC
the form had filled in. Attaching a volume that is already a disk of
the VM, live or in its next start, and adding a NIC with a MAC the VM
has are now refused with the reason.