If a device has been previously installed in the system, had some settings associated with it and then removed - those settings may remain in the system registry, preventing you from doing changes in the future.
This manifests itself often when you add/remove network adapters in your virtual machines (VM) or update their drivers. In such case you may receive an "IP address already assigned to another adapter" error message when trying to configure the same IP on the different virtual NIC. The cause of the error is that an old virtual network adapter settings are in the registry but the device itself is hidden in the Device Manager. Such hidden devices are often referred to as ghosted devices.
It's quite simple to resolve the issue:
Go to Start --> Run, enter "cmd" and press Enter.
At the command prompt window that opens, type set devmgr_show_nonpresent_devices=1 and press Enter. You will not see any confirmation, so make sure you type the above line in exactly.
Now in the same command line window type in start devmgmt.msc and press Enter to launch Device Manager.
In the device manager Select View > Show Hidden Devices.
Expand the Network Adapters tree by clicking the plus sign next to it.
Right-click the grayed-out device, and then select Uninstall. Repeat if necessary.
Close Device Manager.
Note that you will only be able to remove the ghost devices, there will most likely be some hidden system entries that you will not be able to remove (you will be notified that they are needed for system boot).
P.S. Thanks to Matt for the links to the original info: