message: validate course already started
diff: -import { Injectable, NotFoundException } from '@nestjs/common';
  +import {
  + Injectable,
  + NotFoundException,
  + ConflictException,
  +} from '@nestjs/common';
  import { Transactional } from 'typeorm-transactional-cls-hooked';
  import { CourseTakenRepository } from '../repository';
  import { CourseTaken } from '../entity';
  @@ -47,6 +51,12 @@ export class CourseTakenService {
  this.courseService.findById(newCourseTakenDto.courseId),
  ]);
  + const courseTaken = await this.findByUserIdAndCourseId(user, course);
  +
  + if (courseTaken) {
  + throw new ConflictException('This user has already started this course');
  + }
  +
  const lesson: Lesson = await this.lessonService.getByCourseAndSequenceNumber(
  course,
  1,
  