I just edited the thread title for you, using your original post title.
Those entries usually mean that you have devices on your network that have an assigned IP address that doesn't fit that of your router. Potential causes:
1) You have more than one DHCP server on your LAN (which is a big no-no)
2) Those clients were previously connected to a different network, with a different IP
3) You have your wifi wide open, and you have neighbours trying to connect to it (check the MAC addresses in that log to determine if they really belong to one of your devices)