Another device on the network is using your computer’s IP address
This is a rather strange error message, you’ll get the notification “Another device on the network is using your computer’s IP address” and then you’ll lose internet access with the dialog box telling you to try connecting again later. No thanks, we need internet access now! So what causes this and how do we fix it?
It looks to be a problem with the DHCP server, which mistakenly assigns the same IP address to two devices, but don’t blame your router quite yet. For whatever reason it seems that the iPod touch, iPad, and iPhone is often the culprit, it seems these iOS devices like to maintain the same IP address and will attempt to force themselves onto the same IP that they had previously been assigned.
The easiest solution is to just reset the router, but that can be a pain depending on your access to the router itself. If resetting the router is a no go, you can renew your DHCP lease in Mac OS X either through the command line (as the linked article shows) or through the Network settings System Preferences. The other option is to manually set an IP address and have the IP range be far enough apart so that the devices do not conflict.
I assume this is just a bug with how iOS interacts with certain routers DHCP management, it happen to Apple’s own Airport too so perhaps that will spur a patch sooner than later. In the meantime, try one of the fixes, they should get your Mac back online in no time.
Warning about Spoofed MAC addresses
Another possibility (although much less likely) is that someone has managed to spoof your MAC address and IP and is attempting to gain access to your network. I say this is much less likely because assuming you have some reasonable wireless security precautions in place this is very unlikely, there simply aren’t that many people out there with the knowhow to crack into a secured wireless network. The other reason this is less likely is because this problem can be semi-reliably reproduced by simply rebooting an iOS device on a network with only 1 IP assigned (usually 192.168.0.1 or similar). Bottom line: if you see the ‘Another device has your IP’ error message, don’t freak out, it’s probably not a security breach.