“A hard disk with UUID {55b773b8-cbcb-42ea-9a44-1368b30ed823} or with the same properties (‘C:\VBX\Test.vdi’) is already registered.”

You will encounter the above error if you mount virtual hard drive to Virtual Box with the same UUID.
Duplicate/Copy of VDI will not change the UUID.
You need to change it manually by using the following command:

”VBoxManage internalcommands setvdiuuid C:\VBX\Test.vdi”