Doubled Serial #?

I have seen that type of inking before and it seems to be fairly common. I am of the opinion that the ink for the serial number was just running a little low and needed to be refilled soon. If it really was a form of doubling like you see in coins then the seals and the numbers in the four corners would look doubled as well. As far as the black dots it is hard to tell because of the coloring of the picture, but I believe that you are talking about the spots at the bottom of the 9 and the 3 and in a few other places. If it is the same color as the ink then I would say it is just pools of the ink. If is isn't the same color try to scratch a little off with you nail. If any comes off it is debris. If not then take a flashlight and hold it under the serial from the back side. Turn it on and see if the light shines through the dots. If not it is not ink but debris from another source. If light shines through it is most likely ink, but from where I couldn't tell you. Of course I am not an expert so I would wait for another opinion or two .