public void User_Cannot_Be_Removed_From_A_Group_Multiple_Times()
        {
            IUserRepository userRepo = Substitute.For<IUserRepository>();
            IGroupRepository groupRepo = Substitute.For<IGroupRepository>();
            userRepo.UserExists("fred").Returns(true);
            groupRepo.GroupExists("testgroup").Returns(true);
            groupRepo.UserIsAMember("fred", "testgroup").Returns(false);

            UserController controller = new UserController(userRepo, groupRepo);

            var result = controller.RemoveUserFromGroup("fred", "testgroup");

            groupRepo.DidNotReceive().RemoveUserFromGroup("fred", "testgroup");
        }