diff --git a/src/AppBundle/Form/DataTransformer/TagArrayToStringTransformer.php b/src/AppBundle/Form/DataTransformer/TagArrayToStringTransformer.php
index 835a61907..6ed04b40e 100644
--- a/src/AppBundle/Form/DataTransformer/TagArrayToStringTransformer.php
+++ b/src/AppBundle/Form/DataTransformer/TagArrayToStringTransformer.php
@@ -22,6 +22,7 @@
  * See http://symfony.com/doc/current/form/data_transformers.html
  *
  * @author Yonel Ceruto <yonelceruto@gmail.com>
+ * @author Jonathan Boyer <contact@grafikart.fr>
  */
 class TagArrayToStringTransformer implements DataTransformerInterface
 {
@@ -54,7 +55,7 @@ public function reverseTransform($string)
             return [];
         }
 
-        $names = explode(',', $string);
+        $names = array_filter(array_unique(array_map('trim', explode(',', $string))));
 
         // Get the current tags and find the new ones that should be created.
         $tags = $this->manager->getRepository(Tag::class)->findBy([
diff --git a/tests/AppBundle/Form/DataTransformer/TagArrayToStringTransformerTest.php b/tests/AppBundle/Form/DataTransformer/TagArrayToStringTransformerTest.php
new file mode 100644
index 000000000..f77b5986c
--- /dev/null
+++ b/tests/AppBundle/Form/DataTransformer/TagArrayToStringTransformerTest.php
@@ -0,0 +1,142 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Tests\AppBundle\Form\DataTransformer;
+
+use AppBundle\Entity\Tag;
+use AppBundle\Form\DataTransformer\TagArrayToStringTransformer;
+use Doctrine\Common\Persistence\ObjectManager;
+use Doctrine\ORM\EntityRepository;
+
+/**
+ * Tests that tags are transformed correctly using the data transformer.
+ *
+ * See http://symfony.com/doc/current/testing/database.html
+ */
+class TagArrayToStringTransformerTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * Ensures that tags are created correctly.
+     */
+    public function testCreateTheRightAmountOfTags()
+    {
+        $tags = $this->getMockedTransformer()->reverseTransform('Hello, Demo, How');
+
+        $this->assertCount(3, $tags);
+        $this->assertSame('Hello', $tags[0]->getName());
+    }
+
+    /**
+     * Ensures that empty tags and errors in the number of commas are
+     * dealed correctly.
+     */
+    public function testCreateTheRightAmountOfTagsWithTooManyCommas()
+    {
+        $transformer = $this->getMockedTransformer();
+
+        $this->assertCount(3, $transformer->reverseTransform('Hello, Demo,, How'));
+        $this->assertCount(3, $transformer->reverseTransform('Hello, Demo, How,'));
+    }
+
+    /**
+     * Ensures that leading/trailing spaces are ignored for tag names.
+     */
+    public function testTrimNames()
+    {
+        $tags = $this->getMockedTransformer()->reverseTransform('   Hello   ');
+
+        $this->assertSame('Hello', $tags[0]->getName());
+    }
+
+    /**
+     * Ensures that duplicated tag names are ignored.
+     */
+    public function testDuplicateNames()
+    {
+        $tags = $this->getMockedTransformer()->reverseTransform('Hello, Hello, Hello');
+
+        $this->assertCount(1, $tags);
+    }
+
+    /**
+     * Ensures that the transformer uses tags already persisted in the database.
+     */
+    public function testUsesAlreadyDefinedTags()
+    {
+        $persistedTags = [
+            $this->createTag('Hello'),
+            $this->createTag('World'),
+        ];
+        $tags = $this->getMockedTransformer($persistedTags)->reverseTransform('Hello, World, How, Are, You');
+
+        $this->assertCount(5, $tags);
+        $this->assertSame($persistedTags[0], $tags[0]);
+        $this->assertSame($persistedTags[1], $tags[1]);
+    }
+
+    /**
+     * Ensures that the transformation from Tag instances to a simple string
+     * works as expected.
+     */
+    public function testTransform()
+    {
+        $persistedTags = [
+            $this->createTag('Hello'),
+            $this->createTag('World'),
+        ];
+        $transformed = $this->getMockedTransformer()->transform($persistedTags);
+
+        $this->assertSame('Hello,World', $transformed);
+    }
+
+    /**
+     * This helper method mocks the real TagArrayToStringTransformer class to
+     * simplify the tests. See https://phpunit.de/manual/current/en/test-doubles.html.
+     *
+     * @param array $findByReturnValues The values returned when calling to the findBy() method
+     * 
+     * @return TagArrayToStringTransformer
+     */
+    private function getMockedTransformer($findByReturnValues = [])
+    {
+        $tagRepository = $this->getMockBuilder(EntityRepository::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+        $tagRepository->expects($this->any())
+            ->method('findBy')
+            ->will($this->returnValue($findByReturnValues));
+
+        $entityManager = $this
+            ->getMockBuilder(ObjectManager::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+        $entityManager->expects($this->any())
+            ->method('getRepository')
+            ->will($this->returnValue($tagRepository));
+
+        return new TagArrayToStringTransformer($entityManager);
+    }
+
+    /**
+     * This helper method creates a Tag instance for the given tag name.
+     *
+     * @param string $name
+     *
+     * @return Tag
+     */
+    private function createTag($name)
+    {
+        $tag = new Tag();
+        $tag->setName($name);
+
+        return $tag;
+    }
+}