We can have two or more nics with same MAC (physical and alias, two aliases, vlans, bonds...), all assigned to same subnet which IPAM is set to DHCP. Therefore it uses DHCP on smart proxy to create reservations which obviously fails with ugly "conflict" error message. We should warn user or explain the issue ideally before he submits the form. He can either use subnet with internal DB IPAM or None with DHCP boot mode. Another way is not making interfaces managed but then they are ignored in provisioning template.