spring JPA用户，角色身份验证-如何避免重复的角色条目？

@ManyToMany
@JoinTable(name = "user_role",
        joinColumns = @JoinColumn(name = "user_id"),
        inverseJoinColumns = @JoinColumn(name = "role_id"))
private Set<Role> roles;

@Query("SELECT COUNT(u) > 0 FROM User u JOIN u.roles r WHERE u.id = :userId AND r.name = :roleName")
boolean existsByUserIdAndRoleName(@Param("userId") Long userId, @Param("roleName") String roleName);

public void addRoleToUser(Long userId, String roleName) {
    if (!userRepository.existsByUserIdAndRoleName(userId, roleName)) {
        // 添加角色到用户
    }
}