Same IP for different MAC address in DHCP

[iulian_2007]

I have a server with 2 network interfaces in fail-over configuration. We are using DHCP service with "Deny unknown clients" and "Enable Static ARP entries" options checked.
I have declared the first interfaces (MAC, IP, Hostname) and it won't let me declare the second interface (same IP, different MAC, same/different hostname), I get "This Hostname, IP or MAC address already exists." error.
How can this be solved?

[doktornotor]

It cannot be solved in any way.