diff --git a/src/Cheeper/Chapter6/Application/Author/Command/WithDomainEvents/FollowCommandHandler.php b/src/Cheeper/Chapter6/Application/Author/Command/WithDomainEvents/FollowCommandHandler.php
index 5cca266..cf7231f 100644
--- a/src/Cheeper/Chapter6/Application/Author/Command/WithDomainEvents/FollowCommandHandler.php
+++ b/src/Cheeper/Chapter6/Application/Author/Command/WithDomainEvents/FollowCommandHandler.php
@@ -58,10 +58,7 @@ final class FollowCommandHandler
         $follow = $this->followRepository->ofFromAuthorIdAndToAuthorId($fromAuthorId, $toAuthorId);
 
         if (null !== $follow) {
-            throw FollowDoesAlreadyExistException::withFromAuthorIdToAuthorId(
-                $fromAuthorId,
-                $toAuthorId
-            );
+            throw FollowDoesAlreadyExistException::withFromAuthorIdToAuthorId($fromAuthorId, $toAuthorId);
         }
     }
 }
diff --git a/src/Cheeper/Chapter6/Application/Author/Command/WithIdempotency/FollowCommandHandler.php b/src/Cheeper/Chapter6/Application/Author/Command/WithIdempotency/FollowCommandHandler.php
index a1011fa..26ecf55 100644
--- a/src/Cheeper/Chapter6/Application/Author/Command/WithIdempotency/FollowCommandHandler.php
+++ b/src/Cheeper/Chapter6/Application/Author/Command/WithIdempotency/FollowCommandHandler.php
@@ -6,11 +6,11 @@ namespace Cheeper\Chapter6\Application\Author\Command\WithIdempotency;
 
 use Cheeper\AllChapters\DomainModel\Author\AuthorDoesNotExist;
 use Cheeper\AllChapters\DomainModel\Author\AuthorId;
+use Cheeper\Chapter4\Application\EventBus;
 use Cheeper\Chapter4\DomainModel\Author\Author;
 use Cheeper\Chapter4\DomainModel\Author\AuthorRepository;
 use Cheeper\Chapter4\DomainModel\Follow\FollowRepository;
 use Cheeper\Chapter6\Application\Author\Command\FollowCommand;
-use Cheeper\Chapter6\Application\EventBus;
 
 final class FollowCommandHandler
 {
@@ -32,6 +32,7 @@ final class FollowCommandHandler
         $fromAuthor = $this->tryToFindTheAuthorOfId($fromAuthorId);
         $toAuthor = $this->tryToFindTheAuthorOfId($toAuthorId);
 
+        // Idempotency check
         $follow = $this->follows->ofFromAuthorIdAndToAuthorId($fromAuthorId, $toAuthorId);
         if (null !== $follow) {
             return;
diff --git a/tests/Cheeper/Tests/Chapter6/Application/Author/Command/WithIdempotency/FollowCommandHandlerTest.php b/tests/Cheeper/Tests/Chapter6/Application/Author/Command/WithIdempotency/FollowCommandHandlerTest.php
new file mode 100644
index 0000000..56c31bc
--- /dev/null
+++ b/tests/Cheeper/Tests/Chapter6/Application/Author/Command/WithIdempotency/FollowCommandHandlerTest.php
@@ -0,0 +1,178 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Cheeper\Tests\Chapter6\Application\Author\Command\WithIdempotency;
+
+use Cheeper\AllChapters\DomainModel\Author\AuthorDoesNotExist;
+use Cheeper\AllChapters\DomainModel\Author\AuthorId;
+use Cheeper\AllChapters\DomainModel\Author\EmailAddress;
+use Cheeper\AllChapters\DomainModel\Author\UserName;
+use Cheeper\AllChapters\DomainModel\Follow\FollowId;
+use Cheeper\Chapter4\DomainModel\Author\Author;
+use Cheeper\Chapter4\DomainModel\Author\AuthorFollowed;
+use Cheeper\Chapter4\DomainModel\Follow\Follow;
+use Cheeper\Chapter4\Infrastructure\Application\InMemoryEventBus;
+use Cheeper\Chapter4\Infrastructure\DomainModel\Author\InMemoryAuthorRepository;
+use Cheeper\Chapter4\Infrastructure\DomainModel\Follow\InMemoryFollowRepository;
+use Cheeper\Chapter6\Application\Author\Command\FollowCommand;
+use Cheeper\Chapter6\Application\Author\Command\WithIdempotency\FollowCommandHandler;
+use PHPUnit\Framework\TestCase;
+
+final class FollowCommandHandlerTest extends TestCase
+{
+    private const AUTHOR_ID_FROM = '400ea77d-0c8c-44f2-abe8-db05d0852966';
+    private const AUTHOR_ID_TO = '52d8f0b5-544f-46e0-84dc-f8b513391a0e';
+
+    private const USERNAME_KEYVAN = 'keyvanakbary';
+    private const USERNAME_CARLOS = 'carlosbuenosvinos';
+    private const EMAIL_KEYVAN = 'keyvan.akbary@gmail.com';
+    private const EMAIL_CARLOS = 'carlos.buenosvinos@gmail.com';
+
+    private InMemoryAuthorRepository $authorRepository;
+    private InMemoryEventBus $eventBus;
+    private InMemoryFollowRepository $followRepository;
+
+    protected function setUp(): void
+    {
+        $this->authorRepository = new InMemoryAuthorRepository();
+        $this->followRepository = new InMemoryFollowRepository();
+        $this->eventBus = new InMemoryEventBus();
+    }
+
+    /** @test */
+    public function givenTwoNonExistingAuthorsWhenFollowingOneToAnotherOneNonExistingAuthorExceptionShouldBeThrown(): void
+    {
+        $this->expectException(AuthorDoesNotExist::class);
+
+        $this->runHandler(self::AUTHOR_ID_FROM, self::AUTHOR_ID_TO);
+    }
+
+    /** @test */
+    public function givenOnlyFromAuthorIsNonExistingWhenFollowingOneToAnotherOneNonExistingAuthorExceptionShouldBeThrown(): void
+    {
+        $this->expectException(AuthorDoesNotExist::class);
+
+        $this->authorRepository->add($this->buildAuthor(
+            self::AUTHOR_ID_TO,
+            self::USERNAME_CARLOS,
+            self::EMAIL_CARLOS
+        ));
+
+        $this->runHandler(self::AUTHOR_ID_FROM, self::AUTHOR_ID_TO);
+    }
+
+    /** @test */
+    public function givenOnlyToAuthorIsNonExistingWhenFollowingOneToAnotherOneNonExistingAuthorExceptionShouldBeThrown(): void
+    {
+        $this->expectException(AuthorDoesNotExist::class);
+
+        $this->authorRepository->add(
+            $this->buildAuthor(
+                self::AUTHOR_ID_FROM,
+                self::USERNAME_CARLOS,
+                self::EMAIL_CARLOS
+            )
+        );
+
+        $this->runHandler(self::AUTHOR_ID_FROM, self::AUTHOR_ID_TO);
+    }
+
+    /** @test */
+    public function givenTwoAuthorsFollowingOneToAnotherAlreadyWhenTryingToFollowAgainNothingShouldBeHappening(): void
+    {
+        $fromAuthorId = self::AUTHOR_ID_FROM;
+        $toAuthorId = self::AUTHOR_ID_TO;
+        $followId = '51d8ffff-123f-78e1-48fc-f8b513391a0e';
+
+        $fromAuthor = Author::signUp(
+            AuthorId::fromString($fromAuthorId),
+            UserName::pick(self::USERNAME_CARLOS),
+            EmailAddress::from(self::EMAIL_CARLOS)
+        );
+
+        $toAuthor = Author::signUp(
+            AuthorId::fromString($toAuthorId),
+            UserName::pick(self::USERNAME_KEYVAN),
+            EmailAddress::from(self::EMAIL_KEYVAN)
+        );
+
+        $this->authorRepository->add($fromAuthor);
+        $this->authorRepository->add($toAuthor);
+        $this->followRepository->add(
+            Follow::fromAuthorToAuthor(
+                FollowId::fromString($followId),
+                $fromAuthor->authorId(),
+                $toAuthor->authorId(),
+            )
+        );
+
+        $this->runHandler($fromAuthorId, $toAuthorId);
+
+        $this->assertCount(
+            1,
+            $this->followRepository->collection
+        );
+
+        $events = $this->eventBus->events();
+        $this->assertCount(0, $events);
+    }
+
+    /** @test */
+    public function givenTwoAuthorsNotFollowingOneToAnotherWhenFollowingThenTheFollowShouldHappen(): void
+    {
+        $fromAuthorId = self::AUTHOR_ID_FROM;
+        $toAuthorId = self::AUTHOR_ID_TO;
+
+        $fromAuthor = Author::signUp(
+            AuthorId::fromString($fromAuthorId),
+            UserName::pick(self::USERNAME_CARLOS),
+            EmailAddress::from(self::EMAIL_CARLOS)
+        );
+
+        $toAuthor = Author::signUp(
+            AuthorId::fromString($toAuthorId),
+            UserName::pick(self::USERNAME_KEYVAN),
+            EmailAddress::from(self::EMAIL_KEYVAN)
+        );
+
+        $this->authorRepository->add($fromAuthor);
+        $this->authorRepository->add($toAuthor);
+
+        $this->runHandler($fromAuthorId, $toAuthorId);
+
+        $this->assertCount(
+            1,
+            $this->followRepository->collection
+        );
+
+        $events = $this->eventBus->events();
+        $this->assertCount(1, $events);
+        $this->assertSame(AuthorFollowed::class, $events[0]::class);
+    }
+
+    private function buildAuthor(string $toAuthorId, string $userName, string $email): Author
+    {
+        return Author::signUp(
+            AuthorId::fromString($toAuthorId),
+            UserName::pick($userName),
+            EmailAddress::from($email)
+        );
+    }
+
+    private function runHandler(string $fromAuthorId, string $toAuthorId): void
+    {
+        $this->eventBus->reset();
+
+        (new FollowCommandHandler(
+            $this->authorRepository,
+            $this->followRepository,
+            $this->eventBus
+        ))(
+            FollowCommand::fromAuthorIdToAuthorId(
+                $fromAuthorId,
+                $toAuthorId
+            )
+        );
+    }
+}