Looks like RocketUnrealBuildTool is finding ShooterGame.Build.cs and ShooterGame.Target.cs files more than once. These files are auto-discovered in your [ShooterGamePath]\Source folder. Make sure these files appear only once in the Source folder for ShooterGame. Also, these files may have been discovered in your Rocket install folder. Make sure you don’t also have a ShooterGame folder next to your Engine folder in C:\Program Files\Rocket (or whatever your install path is)