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 some

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.