When you use the mod folder, any techs that you create will automatically replace any other techs with the same name in the standard game.
So far so good.
But when you create replacement planetary improvements, they are ADDED to the list of planetary improvements, even if they have the same internal name as one of the old buildings. This means that if, for instance, you wanted to make a mod that increased the cost of all the buildings, you end up with two of each building in the colony list!
Is there any way around this, other than altering the original xml file which rather defeats the purpose of the mod folder?