How do I find out which machine it is from the mac address?

Meant to say if you dont have the MAC address you can check on your DHCP server to see which MAC address has that IP then follow it through the switches.

"arp" will do this to a certain degree.  Here's a couple articles about it:

http://compnetworking.about.com/od/tcpip/f/convertmacipadd.htm
http://www.lockergnome.com/windows/2006/02/23/find-another-computers-mac-address/

How big is your network? For a small network, just look at each device. There are several ways, here's one: Start -> Run -> type "cmd" -> type "ipconfig /all"  (type "exit" to quit)

 Bigger networks may require some snooping or sniffing. If you have multiple segments and routers, see if you can't localize where the duplicate is located. Check the router status to see where things are coming from. If you're not finding it on traditional pc's, don't overlook other potential sources like printers, copiers, portable units, etc.

Cheers ngravatt, saves me typing it out.

binele,

The above will work used it multiple times to track down rogue computers etc.

Cheers
Dan

Thanks guys! Helped alot.