Spring Boot kotlin testing service method with Mockito which saves duplicated names

Question

This is my service logic.

    fun createTags(post: Post, names: List<String>) {
        val tagsToSave = names.stream()
            .distinct()
            .filter { name -> !tagRepository.existsByName(name) }
            .map { name -> Tag(name = name) }
            .collect(Collectors.toList())
        tagRepository.saveAll(tagsToSave)
    }

It's executed before every test method starts.

    val namesWithDuplication = arrayListOf("defaultTag1", "defaultTag1", "tag1", "tag1", "tag2", "tag3")
    fun getMockTagsWithDuplicatedNames() = namesWithDuplication.distinct().map { name -> Tag(name = name) }

    @BeforeEach
    fun init() {
        post = Post(id = 1, title = "test title", body = "test body")
        val tags = arrayListOf(
            Tag(id = 1, name = "defaultTag1"),
            Tag(id = 2, name = "defaultTag2"),
            Tag(id = 3, name = "defaultTag3")
        )
        val postTags = arrayListOf(
            PostTag(id = 1, post = post, tag = tags[0]),
            PostTag(id = 2, post = post, tag = tags[1]),
            PostTag(id = 3, post = post, tag = tags[2])
        )

        postRepository.save(post)
        tags.forEach { tag -> tagRepository.save(tag) }
        postTags.forEachIndexed { index, postTag ->
            postTagRepository.save(postTag)
            post.postTags.add(postTag)
            tags[index].postTags.add(postTag)
        }
    }

Here's my test code.

    @Test
    fun createTagsWithDuplicatedNames() {
        tagService.createTags(post, namesWithDuplication)

        val tags = getMockTagsWithDuplicatedNames()
        verify(tagRepository, times(1)).saveAll(refEq(tags))
    }

I expect this test won't succeeded because getMockTagsWithDuplicatedNames() returns tags("defaultTag1", "tag1", "tag2", "tag3"). But createTags() saves tags("tag1", "tag2", "tag3") because post already saved "defaultTag1" by @BeforeEach method.

        verify(tagRepository, times(1)).saveAll(refEq(tags))

But mockito actually doesn't save tags, so the test doesn't fail.

How should I handle it to make it fail?