I had this problem when I got my new Wi-Fi dongle and have seen a few people with the same issue. Basically when I have one interface configured and want to swap to the other one, it throws up this error:
RTNETLINK answers: File exists
Failed to bring up eth0
or something similar.
auto lo iface lo inet loopback auto eth0 iface eth0 inet static address 192.168.1.2 netmask 255.255.255.0 gateway 192.168.1.1 iface wlan0 inet static address 192.168.1.3 netmask 255.255.255.0 gateway 192.168.1.1