1) Shut down the virtual machine you would like to copy
2) In File > Virtualdiskmanager, select the virtual machine disk image you would like to copy, and press the Release button and DELETE it from the menu BUT keep the image on the harddrive.
3) In a terminal window, issue following command (see virtualbox user manual):
vboxmanage clonevdi /directory/image1.vdi /directory/image2.vdi
4) In File > Virtualdiskmanager, add the new disk image you've created in step 3.
5) In the main virtualbox window, press the New button to create a new virtual machine, and link it to the new disk image you've created.