>I am switching from one old net card (3C503) to another (WD8013EPC,
>made by SMC) so I inserted the new one along with the old one and
>configured the kernel with it.
>
>But I cannot ifconfig it:
>
>% ifconfig we1 192.168.0.8 netmask 0xffffff00 media 10base2/BNC
>ifconfig: SIOCAIFADDR: File exists

>What file is it referring to?

Probably to the route entries for the same subnet (92.168.0.0/24)
already set for your ec0 interface, assuming you use the same subnet
for both interfaces.  Possibly to the ARP entry for 192.168.0.8, if
you already used that address on ec0.