Early Roman coins were struck by hand, rather than being cast. First an engraver created two punches out of bronze; one for the obverse of the coin or front side and the other for the reverse or back side of the coin. The denarius was made by place a small silver disk, often varying in size & thickness, but not weight, between these two bronze punches and then striking it with a hammer. This forced the images on the front and back of each coin. It's easy to see with this method why Roman coins were often struck off-center with part of the design occasionally lost off the edge of the coin. As for double stamped 'fronts' like this Blasio [seen below]; this occurs because a coin was struck and the top punch was taken off. However, the new coin was accidentally left on the bottom punch. A piece of flat silver was then simply placed upon it. The top punch was placed upon the flat silver and everything was struck with the hammer. The effect is that the front of the coin looks perfect but the back has a mirror image of the front of the previous coin.