You may get the following error when importing existing image to the VirtualBox:
A hard disk with UUID {e9758c99-e50c-41c7-877f-f03c1454d136} or with the same properties (‘image.vhd’) is already registered.
This happens when you make copy of a virtual image and want to add it to VirtualBox, but the original image is also registered. To change the UUID use the VBoxManage utility installed with VirtualBox.

VBoxManage.exe internalcommands sethduuid image.vhd