Is it the router IP address.. or your device's IP address.. that is being used by another? If it is the device IP address being reused then that's more than likely not a problem. However, it is your router's IP address then you shuld look into it a little more.
Have you physcially assigned an IP address for your device to use? Usually if you enable your router for Automatic (DHCP) IP address distribution, your device's IP address on your local area network will change without you knowing.