I believe it means you have two computers trying to use the same IP address at the same time. The Mac you are using and the machine with MAC address 00:23:bc:91:fa:2f.
Open a terminal window and type "ifconfig" (without the quotes) and see what IP address (probably the same as the one given above) and MAC address you have. If you have other computers using the same network check them and see what their MAC address is until you find the one with the MAC address above. Then find out why it is using that same IP address.
This issue usually happens on a DHCP network when someone statically assigns an IP address in a DHCP scope and then the DHCP server gives the same IP address out to another machine. Only one machine can use an IP address in the same subnet at the same time.
If you find a machine with this IP address statically assigned, remove it and set the machine up for DHCP. If you have statically assigned this IP address to your Mac, remove it because DHCP will provide one for you automatically.