ERROR: duplicate key value violates unique constraint "user_username_key" Detail: Key (username)=(test) already exists

I have a problem trying to delete, then save a user in an application using the PostgreSQL database. Here is my controller:

@PostMapping
    public String addCalories(@RequestParam Integer consumedKcal, HttpServletRequest request){
        User u = (User) request.getSession().getAttribute("u");
        User newUser = this.userService.save(u.getUsername(), u.getPassword(), u.getAge(), u.getGender(), u.getHeight(), u.getWeight(), u.getGoal(), u.getActivity_rate(), u.getKcal_needed(), u.getBmi(), consumedKcal);
        return "redirect:/home";
    }

Here is my service method:

@Override
@Transactional
    public User save(String username, String password, Integer age, String gender, Float height, Float weight, String goal, String activity_rate, Integer kcal_needed, Float bmi, Integer kcal_consumed) {
        this.userRepository.deleteUserByUsername(username);
        User u = new User(username, password, age, gender, height, weight, goal, activity_rate, kcal_needed, bmi, kcal_consumed);
        return this.userRepository.save(u);
    }

The repository:

public interface UserRepository extends JpaRepository<User, Integer> {

    User findUserByUsername(String username);

    void deleteUserByUsername(String username);
}

The entity:

@Data
@Entity
@Table(name = "user_app")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer username_id;
    private String username;
    private String password;
    private Integer age;
    private String gender;
    private Float height;
    private Float weight;
    private String goal;
    private String activity_rate;
    private Integer kcal_needed;
    private Float bmi;
    private Integer kcal_consumed;
    private Integer water_consumed;
}

The JPA method delete works fine, it deletes the user. But I get this error duplicate key value violates unique constraint "user_username_key" Detail: Key (username)=(test) already exists. I don't know how to solve it. Any help is welcome.