Presumably both computers are networked. The normal setup has IP addresses being assigned to each device by the router. If the router's address is 192.168.0.1 then it usually assigns addresses 192.168.0.2 then 192.168.0.3 etc. to devices as they connect to the router. However, it is also possible to setup computers to have a permanently assigned IP address. So if you assign 192.168.0.2 to the laptop, it will work with the router. On another occassion, the router is also able to assign that same IP address to your other computer. If this has happened and you then add the laptop to the network they will have the same IP address, hence the error message.
This can also occur if there are two network cards in one machine. If this is the case disable one of the cards if it is not being used. To do this open Network and dial up connections, right click the unused card as select disable.
You might have manually configured the ip addresses. Try right clicking the network icon in the taskbar and 'open network connections' then under the 'general' tab select TCP/IP and click 'properties'. Make sure the boxes are checked to 'obtain IP address automatically'.
This might be the problem.
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