I just captured a core world from the Mimot Brotherhood (I have no idea whether they colonized it or whether they captured it from someone else) and, as shown in the first screen snapshot below, it has 3 Stellar Forges. The popup says that the Stellar Forge is a "Civilization Achievement" which, I though, meant that you can only build one in your civilization. However, here is a core world that has 3.
The second screen snapshot shows that the manufacturing for this core world is benefitting from all 3 Stellar Forges, 2 Durantium Processors, and 2 Towers of Endless Night. What is going on? Do one or more civilizations have the ability to build more than one of the same improvements that I am only able to build one of?
If it makes a difference, I am playing on Godlike difficulty.