As anyone in the printing business can relate, it's been quite easy for many years to duplicate the image on currency. The whole deal seen in movies about "getting the plates" and all is totally bogus or at least has been now for a good many years. Anyone with a camera, a way to develop the image and burn a plate, and then slap it on a press and run it, which any Quick Copy joint could do in no time, could easily dyplicate the image of the $$.

Like FA mentioned though, the subtrate (paper/cloth material money is printed on) and also the qualities specific to the ink used to print currency are the hard parts to duplicate. Since the US currency finally stepped up to the plate and started using water marked stock in the process, this also added another layer of protection to the currency that made it harder still to replicate.

No matter what though, if humans can create it, somehow or some way, other humans can also duplicate it