You can either have the installation that comes with Ubuntu or the Oracle’s release.
If you are using the Oracle’s release (https://www.virtualbox.org/wiki/Downloads) then everything you need to do is right-click on your machine and “clone”, on the Virtualbox’s management interface.
However, if you are using the version that comes in Ubuntu’s repository, then you need to do it manually, and it is a little bit more complicated than just copying the VDI file. Because of the UUID of the VDI, that ID must be different for each machine. But it’s easy. Let’s say your image is Ubuntu1.vdi and you want an Ubuntu2 machine:
vboxmanage clonevdi ../Ubuntu1/Ubuntu1.vdi Ubuntu2.vdi
Now, on the Virtualbox’s user interface, you can create the new machine with an existing disk. It’s almost done.
Now, if you start your machine, the network interface will have the same MAC address and (for a reason that I don’t understand) it won’t work. All you have to do is (on Virtualbox’s interface) go to the settings of your new machine, then on network, advanced and change the MAC address to anything else valid.
Supposing your guest system is also Linux (Ubuntu in my case), start your machine and you will notice it’s having trouble setting up the network. You need to edit the file /etc/udev/rules.d/70-persistent-net.rules . There will be two lines effectively working. Delete the line about “eth0”. On the line about “eth1”, replace the “eth1” with “eth0”.
Done, reboot and you are good to go.