Looks for me like a network issue for me...
try flushing your arp-list, don't know the syntax on mac, on windows it's
arp -d *
Removes all entries in the arp-list
Or alternatively you can set the same MAC address for both mashines.
I'll see about clearing any connection between IP and MAC addresses, but if the Admin Console is using MAC addresses, that defeats the purpose of asking for an IP address or DNS name.
We also had similar issues when we tried to install a client's server. The best solution we came up with was to install vmWare ESXi (free version) on both machines and have the same two virtual machines running on each system, so on most days both machines were being used, but in a 'machine failure' mode, either machine could quickly perform in the 'two machine configuration' with having to change IPs, re-run the deployment assistant, or any other trickery.... just don't launch all fout vms at the same time!!! (mass chaos and conflicts).
Before someone corrects me, vmWare start offically supporting Apple hardware in Aug of 2011; so this will work in an xServe enviroment...
Another option you may wish to look at is doing snapshot backups, which capture the 'state' of the machine in a second or so. If you used something like Symantec Netbackup (Windows) your backup machine would be a mirror image of the original--- IP addresses, MAC addresses, files, etc...