In some cases, namely PXE Boot, the GUID is based on the MAC. The MAC is supposed to be unique – but this actully isn’t always so. This, however, should be rare. If you move the NIC around or have a failure of the installation process and start over, you can have a problem with the GUID already being registered in AD. This issues is alluded to in the following article:
In the article (at the bottom) it discusses how the duplication can occur if you move NICs. However, if there is a failure in the initial installation and the installation is started over, you can have the problem as well. This can be a timing issues as well – in other words an installation fails and the deletion of the GUID from AD doesn’t occur immediately and the installation is re-tried. You get the warning, but if you wait a moment you can retry without a problem. Occassionally, I have heard that AD edits for the object in question are necessary (ADSIEdit works). I have never personnally experienced having to do an edit for this purpose, but understand it to be possible.