I can understand the researched twice thing, I just think its a bit intelligent. Firstly the two techs are quite diffrent so they should have their own discriptions, icon and everything. Secondly its the only place in the entire tech tree currently proposed for something like this. Refinements are planned to be added, but I'd hardly think builing entire moons and planets is a refinement
and finally even if you have one tech researched twice, for game balance issues its two techs so it still takes up a slot that could be used on something else.
And while I agree it makes little sense if you can build a moon, but can't build a big station, but that doesn't mean you need a specific tech for it. It could just be pushed into the background, e.g. once stations start getting big the architects discover this problem, and fix it themselves without ever needing a massive government funded reaserach
If we can think of something that will fill that particular slot in the tech tree better I say drop self gravitating structures and put the new thing in, and yes, I do think that modular space stations would fit better because:
1) As history shows, modular design is a major improvement. Much more useful than just makeing things bigger.
2) Its a very down to earth sort of tech (I appreciate the irony), modular space stations rather showing that space is becoming more wondrous like Self Gravitating Structures dose the opposite, showing that space is becoming more and more part of daily life.