If you began to implement Google’s API V3 of their In App Billing Service, and switched to android-inapp-billing-v3 made by anjlab, you might get this error message
The reason this is happening is because you have the same library in there twice. Once from before, and again when his library adds the same files.
The solution is simple, delete the aidl folder in your <project>/app directory. Clean, then rebuild and it will work!