I took the article at its word that it was a duplication! You are right, those are other possibilities.
There are a small number of publications on IJV duplications, and none of them mention the patient as being an MS patient. Most have the veins staying separate after they split but here is one where the two IJVs merged back into one IJV:
http://www.ajnr.org/content/25/8/1433.fullQuote:
The appearance of an internal jugular vein on CTA images is characteristic. The vein begins its course with a single lumen (A, B). It then bifurcates forming two distinct lumena (C). As the vein continues caudally, it once again becomes a single vessel (D, E). Panel F shows the fenestrated internal jugular vein reformatted in a single sagittal, oblique plane.
Quote:
The internal jugular vein is the great collecting vein of the cranium, face, and anterior neck. It is the continuation of the sigmoid sinus beginning at the jugular foramen, where it drains most of the venous blood from the brain as well as CSF. The internal jugular vein continues inferiorly from the skull base until it joins the subclavian vein to form the brachiocephalic vein.
Fenestration of the internal jugular vein is uncommon. There have been five previously published reports of seven cases of a fenestrated internal jugular vein (2–6). One study estimated that fenestration of the internal jugular vein occurs in as much as 0.4% of the population (2).