You did something. Maybe you replaced a network adapter on a physical server. Maybe the server is a virtual machine, and you moved the vm to a new host. Maybe you restored an OS onto new hardware, or moved an OS on the SAN from one piece of server hardware to another. The result is that your network adapter is now set to DHCP.
You go into your Windows Server 2003 control panel, network connections, and pull up properties of the network adapter. Open properties of Internet Protocol (TCP/IP) and set the IP address. When you click Ok, you get:
The IP address a.b.c.d you have entered for this network adapter is already assigned to another adapter 'Some Ethernet Adapter' on this computer. If the same address is assigned to both adapters and they become active, only one of them will use this address. This may result in incorrect system configuration. Do you want to fix the problem by entering a different IP address for this adapter in the list of IP addresses in the advanced dialog box?
You check the control panel and do not see any other network adapters. The reason is that the adapter is hidden from network connections control panel once it is removed. However, it still contains its IP address.
The resolution is to turn nonpresent devices on and remove the network adapter from device manager:
When Device Manager opens, select View > Show hidden devices. Then browse to network adapters and remove all adapters. Right-click the top-node and scan for new hardware. Your new network adapter will then reinstall itself. Now you will be able to set the IP address without getting the error message.
If this happens frequently, such as if you have a dynamic environment were Windows Server 2003 instances are getting shuffled around regularly, you can set nonpresent devices to be always shown. Add devmgr_show_nonpresent_devices=1 to the System Variables (My Computer, Properties, Advanced, Environment Variables, System Variables).