getting a weird error...

Hmm sounds like your DHCP isn't getting renewed every time you log on to the internet.

 

Is this your own network or are you hoping onto someone else's network? (not saying this to be mean) Are there other computers or devices that use this network. When you close down for the night another device could have done a renew getting the same IP that you had on your machine, and when you open up it will try to connect to the same IP causing that error. You can go into System Preferences>Network>Airport and do a Renew IP that will clear out the old and give a new IP address.. And the in used by XX.XX.XX.XX.XX.XX is the MAC address of the device that is using that IP. you can try to track down what that device is.

If this is your wireless network make sure that it is secure. some one else could be popping onto your network.

 

System Preferences -> Network -> Airport -> Advanced -> TCP/IP

What is selected in this menu?

 

Yeah if it's an unsecured network, it could be that somebody's hopping onto your network without your knowledge. If your lease expired while your Mac is sleeping and the router gives the new machine your old IP address, it could cause an IP conflict when your Mac wakes up? This is purely speculation so if I'm wrong somebody please correct me. I'd suggest WPA2 security for your network though.

 

You should secure it otherwise intruder is going to take your wireless internet. Try give your network a WPA password by going to your router's setting and after that unplug your modem's power cord and re-plug it back in and everything should be back to normal.

 

This is very plausible and you should definitely secure your wireless network as someone can easily gain access to personal information.