Update January 2014: there is a dedicated Duplicate IP Address Scanning Tool in NetScanTools Pro.
I've mentioned before how due to past problems with online games sites I have my son use a Linux distribution called Knoppix 6.0.1 that runs from a CD inside a Microsoft Virtual PC 2007 virtual machine. Well due to a problem with a DHCP server, I found that Knoppix was taking the same IP address as an HP Laser Printer. I had been having trouble with the printer on the weekend - it decided on it's own to change it's fixed IP address.
So I decided to use the situation as a real world demonstration of how to find a duplicate IP address. This can be done from NetScanTools Pro using the ARP Ping Tool. Since I had my suspicions about the printer, I used the printer IP. The video shows the results quite clearly.
In NetScanTools Pro v11 we will be introducing a tool to scan the whole subnet for duplicate IPs, not just one at a time.