Well I played through the entire tech tree for the Altarians and encountered several Colony Improvements which are bugged. I've searched the forum, but found no existing thread about it so I made one even the issues may be known already:
- Missionary Center, Intimidation Center, Preparedness Center (They are marked as Colony-Unique, but I can build them multiple times on each colony, sometimes especially if they are placed on a tile with a bonus, but most of the time also without a tile bonus, there's no clear pattern to why some colonies are able to have them more than once and some not.)
- Restaurant of Eternity (I accidently teared it down because I selected the wrong tile and couldn't build it anymore after that point as it didn't show up in the list)
- Precursor Souvenir Shop (Same as with the ideology buildings, sometimes it can be build multiple times even it is marked as Colony-Unique. Also no clear pattern to where and why.)
- Drathian Temple (Same as with the ideology buildings, sometimes it can be build multiple times even it is marked as Colony-Unique. Also no clear pattern to where and why.)
- Planetary Defense System (Marked as Colony-Unique, but once upgraded to the Planetary Defense Dome it can be build again... don't know if that's intended? And if it is it should be slipstreamed, because once the Planetary Defense Dome is built there's basically no need for the Planetary Defense System anymore.)
- Research Cloisters (Marked as Player-Unique (& Colony Unique), but once built and upgraded to the Quantum Research Foundation it can be build again on another planet, also don't know if that's intentional behavior, because I think it should also be fixed/slipstreamed.)
- Terraforming Improvements (Altough only a minor problem, some of them show up even if there's no tile of sufficient quality available to be upgraded anymore. Would be nice if they are not shown in the list in the case they actually can't be used anymore, because they always catch my attention. 
)
Also another minor suggestion would be to make the improvement list better ordered. Something like ordered by category (Manufacturing, Research, Population,...) because even if most of them are following the ordering already, some of them don't and then you have to scroll up and down like crazy to find the one you want.
I may update the list if someone has to contribute other improvements not listed up there, because it would take quite some time to play and find the improvement bugs with the other civilizations.
