I was trying to restore a .vdi disk image that had been manually backed up (i.e. copied). The problem when trying to add this copied disk image to a VM is that the UUID conflicted with a virtual disk that already existed.

The easiest solution is to change the UUID of the either disk image. The following command worked for me. Note the "/" in the path, I was using a Linux laptop so for a Windows based system you should use "\" in the path instead.