| Put it in this folder: C:\Program Files\Stardock\TotalGaming\GalCiv2\Mods\Default\Data\PlanetImprovements |
Since that mod is the complete PlanetImprovements.xml file with additions, it should go in the \Data folder, not the \Data\PlanetImprovements folder.
If it goes in the \PlanetImprovements folder there will be two of everything (except the additional items, which only occur in the mod file).
It is really meant to be a replacement for the original PlanetImprovements.xml, not an addition to it. And that is how those mod folders work - files in the \Data folder replace the original, and files in the specific subfolders add their contents to the main file.
