>Too bad they didn't say anything about dual citizens
How could they? At the time of the Constitutions adoption, the only country on Earth with "citizens" was the USofA. The people of Europe were subjects of the crown. China, Japan, India were feudalistic, where most people were property of the nobles who held title to the land.
Also, the theorists of "citizen" at that time believed the individual was sovereign, not the state. As subject gave way to citizen across the world, people would be free to move about to other Republic's and pledge their allegiance to their new home. The idea that one could have more than one allegiance at a time didn't compute; by definition a citizens national allegiance and loyalty and honor was indivisible, thus a dual citizen COULD NOT EXIST. For the Constitutions authors, dual citizenship was impossible, so why discuss it.