It seems like a rule designed by journal editors to protect their turf, or PhD committees to make it easier to count original works towards degree requirements. What could possibly be the justification?
It also protects readers who may encounter the second years latter and not realize it is the same data and thus they think it is a second study reenforcing something. If they are experts in the field they likely know, but if this is a case where a different field overlaps they will want to have citations without as much knowledge of what is important.