As I understand it, at least in Unity a vertex has one UV (per texture) - so if I mark a seam in Blender, there would be 2 UV coordinates for a particular vertex. Does that mean it duplicates the vertex, as it does when marking a sharp edge?
Yes. The third thing that will do this is using vertex color "masks". It's not the vertex itself that needs to be duplicated, but that's the only choice, when you need more than one data type in one location. UV, UV2, Normal, and Color are those possible data types.