this can be caused by a variety of problems. What it amounts to is that two computers on your LAN are trying to use the same IP address.
The biggest cause of this I see is when someone has multiple machines booted up on their lan and reboots their router, but a switch or wifi does not get rebooted. Computers connected to the router will re-dhcp normally, but computers connected to a powered on switch or wifi will NOT, and will continue to use the IP address the router gave them earlier. The router will reset its DHCP table when it reboots, and begin handing out addresses from the start again. So if a machine hanging on a switch has 192.168.1.101, and 101 is the start of the pool, the first machine to dhcp off the router when it reboots will also attain 101, and now you have two machines with the same IP address on the lan, and one or both will pop up a warning.
The easiest way to fix this is to power cycle ALL your network hardware. Or you can run around and reboot all your computers, whatever's easier.
There are other less common ways to trigger this message. Static mapping your computers will do it if you don't pay attention to keeping the static and dhcp ranges separate. More than one router on your LAN handing out DHCP leases will also do it. It can be tricky scouring a large building to find the rogue router someone has connected backwards (LAN port on their router from home to jack on the wall) in their office behind their desk. Networked printers are often configured static and can be misplaced inside the dhcp pool.
You can look at the mac address that is listed and get on the internet and look up the prefix to see what sort of hardware you are hunting for. For example, you may find you are looking for a certain model of NIC that only comes with windows drivers. Or even a specific model of printer. Or a range that Apple is registered to. nmap can be slightly helpful with this too, you can disconnect the computer with the message from the network and use nmap to do an OS identification on the IP and see if it can get you some information. nmap can get very specific, "windows NT, service pack 3" etc.
Edited by Virtual1 (04/05/12 02:53 PM)
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