There are several old threads on this forum from 2010 and 2011 that discuss it.
On a home network, it generally turns out to be a harmless notification, possibly due to some other device on your network (e.g., cell phone, video game console) sending an address resolution protocol (ARP) reply without your Mac having made an ARP request.
If the IP address corresponds to one of your own devices, it's unlikely that a hacker has gained access to your network and is spoofing it. If the notifications bother you, you can turn off the notification itself (while allowing the software to continue to monitor for that vulnerability).
