It's a bit frustrating to not have clear and distinct visible cues for all the improvements, so you could at a glance see if it's a unique building (one per player or galaxy) or simply a one-per-planet type of improvement.

So here's some suggestions to possible solutions:

• Add a distinct row for unique improvements, so any player- or galaxy-limited building would end up on a separate row with a small header to make them distinct.
• Add a visual tag or marker on the icon for unique buildings, making them easy to spot in the list of normal improvements.