If you don't have the vboxguestadditions.iso for your virtual machine, you can download it from here:
http://download.virtualbox.org/virtualbox/2.0.4/VBoxGuestAdditions_2.0.4...
To clone a Virtual Machine in VirtualBox, first go to:
/home/[username]/.VirtualBox/VDI
in the console, use command:
(don't forgot to change the virtual disk name to your existing virtual disk name.)
VBoxManage clonevdi vm1.vdi vm2.vdi