Question : How do you find a duplicated MAC address (spoofing)?

In trying to figure out why our network traffic seems to be up, I've installed NTOP on one pc and put it off of a hub to monitor the traffic.  One of several things that NTOP reports on is duplicate MAC addresses.  Several of our pc's have been red flagged as high risk because NTOP found duplicate MAC addresses which might indicate spoofing.
 
If indeed something/someone is spoofing, how to I track this down?  How do I find out if it's really a MAC address being spoofed and then find the duplicate?
 

Answer : How do you find a duplicated MAC address (spoofing)?

              *When you run network detection, which lists mac addresses, spoofed MACs would be easily recognized if spoofer is not a prof.
                For ex
               00-50-FC-F7-27-FD is a normal MAC address at first look. Spoofers usually spoof to thing like following
               00-12-34-AB-CD-56 and this is a suspicious MAC.
             
             *For more details please check this link
http://en.wikipedia.org/wiki/IP_Traceback