Duplicating and copying a Virtual Machine

How To: Properly Backup a VirtualBox Machine (.VDI)
This is the proper way to backup your VirtualBox Machine:
VBoxManage clonevdi source destination
Example:
VBoxManage clonevdi ~/.VirtualBox/VDI/WindowsXP.vdi ~/WindowsXP_Backup.vdi
NOTE:  Although I’m not specifically sure, sometime after Version 2 of this software, the clonedvi command has been replaced with clonehd (see page 108 of the VirtualBox Manual), however, clonedvi will still work as they kept the backwards compatibility.
Then, wait for it to complete. It may take a while depending on the size of your .vdi file (or how much space you allocated towards your virtual machine).
What this actually does is create a new UUID (Universal Unique Identifier) for the cloned VM.  This way, you won’t end up with a message similar to this:

A hard disk with UUID {4d749826-6a3f-43ff-90af-42618783bd3a} or with the
same properties (’/home/martin/.VirtualBox/VDI/test.vdi’) is already
registered.

• Shrinking a Dynamic VirtualBox Disk Image
• How To: Install VirtualBox on Fedora 11
• How To: Install VirtualBox on Ubuntu 8.04LTS

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To duplicate a VBox machine:
VBoxManage clonevdi Master.vdi Clone.vdi
and the (undocumented) function
cp Master.vdi Clone.vdi
VBoxManage internalcommands setvdiuuid Clone.vdi
which actually staffs the clone with a new SID in one step.

Must read:
How to copy (clone) a VirtualBox VDI in Ubuntu

Cloning and Copying VirtualBox virtual machines
Command line of VBox in Windows XPOpen a command prompt, cd to the c:\program files\innotek directory and launch the VBoxManage command from there. You could also add the innotek prog files directory to your path if you wanted to run the VBoxManage from any directory.