This simple technique shows you how you would duplicate floppy disks in a GNU/Linux system using dd. This technique is not as useful as it used to be but can still be used for creating an image of a cd (although that is best done through the cd burning program).
This information has been taken from the Linux Online Classroom, see  in the Bibliography for further details.
$ dd if=/dev/fd0 of=floppy-image $ dd if=floppy-image of=/dev/fd0
The first dd makes an exact image of the floppy to the file floppy-image, the second one writes the image to the floppy. (The user has presumably switched the floppy before the second command. Otherwise the command pair is of doubtful usefulness).