Hey Marrv, I believe I have a solution to your problem as I had an identical issue. Make sure in your KSP GameData folder, you do not have a folder also called GameData with the KW rocketry mod inside of it. I accidently made the mistake of having KW rocketry in both the main folder and putting a sub GameDate folder inside of it with the mod. This duplication caused the double parts and other errors with the KW part I am fairly certain.