You can duplicate hard disk image files on the same host to quickly produce a second virtual machine with the same operating system setup. However, you should only make copies of virtual disk images using the utility supplied with VirtualBox; see Section 8.24, “VBoxManage clonemedium”. This is because VirtualBox assigns a unique identity number (UUID) to each disk image, which is also stored inside the image, and VirtualBox will refuse to work with two images that use the same number. If you do accidentally try to re-import a disk image which you copied normally, you can make a second copy using VirtualBox's utility and import that instead.
Note that newer Linux distributions identify the boot hard disk from
the ID of the drive. The ID VirtualBox reports for a drive is determined
from the UUID of the virtual disk image. So if you clone a disk image and
try to boot the copied image the guest might not be able to determine its
own boot disk as the UUID changed. In this case you have to adapt the disk
ID in your boot loader script (for example
/boot/grub/menu.lst). The disk ID looks
like this:
scsi-SATA_VBOX_HARDDISK_VB5cfdb1e2-c251e503
The ID for the copied image can be determined with
hdparm -i /dev/sda