diff --git a/core/modules/forum/config/optional/taxonomy.vocabulary.forums.yml b/core/modules/forum/config/optional/taxonomy.vocabulary.forums.yml
index b5c51eb..ef2e54a 100644
--- a/core/modules/forum/config/optional/taxonomy.vocabulary.forums.yml
+++ b/core/modules/forum/config/optional/taxonomy.vocabulary.forums.yml
@@ -9,3 +9,4 @@ vid: forums
 description: 'Forum navigation vocabulary'
 hierarchy: 1
 weight: -10
+prevent_duplicates: false
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Vocabulary/VocabularyResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Vocabulary/VocabularyResourceTestBase.php
index abada74..dc9f8cc 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Vocabulary/VocabularyResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Vocabulary/VocabularyResourceTestBase.php
@@ -56,6 +56,7 @@ protected function getExpectedNormalizedEntity() {
       'description' => NULL,
       'hierarchy' => 0,
       'weight' => 0,
+      'prevent_duplicates' => FALSE,
     ];
   }
 
diff --git a/core/modules/taxonomy/config/schema/taxonomy.schema.yml b/core/modules/taxonomy/config/schema/taxonomy.schema.yml
index efa08e1..e3c6c82 100644
--- a/core/modules/taxonomy/config/schema/taxonomy.schema.yml
+++ b/core/modules/taxonomy/config/schema/taxonomy.schema.yml
@@ -33,6 +33,9 @@ taxonomy.vocabulary.*:
     weight:
       type: integer
       label: 'Weight'
+    prevent_duplicates:
+      type: boolean
+      label: 'Prevent duplicates'
 
 field.formatter.settings.entity_reference_rss_category:
   type: mapping
diff --git a/core/modules/taxonomy/src/Entity/Term.php b/core/modules/taxonomy/src/Entity/Term.php
index 0c3a0eb..e89cda6 100644
--- a/core/modules/taxonomy/src/Entity/Term.php
+++ b/core/modules/taxonomy/src/Entity/Term.php
@@ -128,7 +128,8 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
         'type' => 'string_textfield',
         'weight' => -5,
       ))
-      ->setDisplayConfigurable('form', TRUE);
+      ->setDisplayConfigurable('form', TRUE)
+      ->addConstraint('TermName', []);
 
     $fields['description'] = BaseFieldDefinition::create('text_long')
       ->setLabel(t('Description'))
diff --git a/core/modules/taxonomy/src/Entity/Vocabulary.php b/core/modules/taxonomy/src/Entity/Vocabulary.php
index a2d7eef..9bde33e 100644
--- a/core/modules/taxonomy/src/Entity/Vocabulary.php
+++ b/core/modules/taxonomy/src/Entity/Vocabulary.php
@@ -43,6 +43,7 @@
  *     "description",
  *     "hierarchy",
  *     "weight",
+ *     "prevent_duplicates",
  *   }
  * )
  */
@@ -89,6 +90,13 @@ class Vocabulary extends ConfigEntityBundleBase implements VocabularyInterface {
   protected $weight = 0;
 
   /**
+   * Prevents to create terms with the same name.
+   *
+   * @var bool
+   */
+  protected $prevent_duplicates = FALSE;
+
+  /**
    * {@inheritdoc}
    */
   public function getHierarchy() {
@@ -120,6 +128,13 @@ public function getDescription() {
   /**
    * {@inheritdoc}
    */
+  public function preventDuplicates() {
+    return $this->prevent_duplicates;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public static function preDelete(EntityStorageInterface $storage, array $entities) {
     parent::preDelete($storage, $entities);
 
diff --git a/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraint.php b/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraint.php
new file mode 100644
index 0000000..4bf8fbe
--- /dev/null
+++ b/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraint.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace Drupal\taxonomy\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Checks if a value is a valid term name.
+ *
+ * @Constraint(
+ *   id = "TermName",
+ *   label = @Translation("Term name", context = "Validation"),
+ * )
+ */
+class TermNameConstraint extends Constraint {
+
+  public $message = 'The term %value already exists in vocabulary %bundle.';
+
+
+}
diff --git a/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraintValidator.php b/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraintValidator.php
new file mode 100644
index 0000000..11451ae
--- /dev/null
+++ b/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraintValidator.php
@@ -0,0 +1,57 @@
+<?php
+
+namespace Drupal\taxonomy\Plugin\Validation\Constraint;
+
+use Drupal\Component\Utility\Unicode;
+use Drupal\Core\Entity\ContentEntityInterface;
+use Drupal\taxonomy\Entity\Vocabulary;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Validates the TermName constraint.
+ */
+class TermNameConstraintValidator extends ConstraintValidator {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($items, Constraint $constraint) {
+
+    if (!$item = $items->first()) {
+      return;
+    }
+
+    $field_name = $items->getFieldDefinition()->getName();
+    /** @var ContentEntityInterface $entity */
+    $entity = $items->getEntity();
+
+    if (!Vocabulary::load($entity->bundle())->preventDuplicates()) {
+      return;
+    }
+
+    $entity_type_id = $entity->getEntityTypeId();
+    $bundle_key = $entity->getEntityType()->getKey('bundle');
+    $id_key = $entity->getEntityType()->getKey('id');
+
+    $value_taken = (bool) \Drupal::entityQuery($entity_type_id)
+      // The id could be NULL, so we cast it to 0 in that case.
+      ->condition($id_key, (int) $items->getEntity()->id(), '<>')
+      ->condition($field_name, $item->value)
+      ->condition($bundle_key, $entity->bundle())
+      ->range(0, 1)
+      ->count()
+      ->execute();
+
+    if ($value_taken) {
+      $this->context->addViolation($constraint->message, [
+        '%value' => $item->value,
+        '%bundle' => $entity->bundle(),
+        '@entity_type' => $entity->getEntityType()->getLowercaseLabel(),
+        '@field_name' => Unicode::strtolower($items->getFieldDefinition()->getLabel()),
+      ]);
+    }
+
+  }
+
+}
diff --git a/core/modules/taxonomy/src/VocabularyForm.php b/core/modules/taxonomy/src/VocabularyForm.php
index 1868ddc..f96e831 100644
--- a/core/modules/taxonomy/src/VocabularyForm.php
+++ b/core/modules/taxonomy/src/VocabularyForm.php
@@ -73,6 +73,11 @@ public function form(array $form, FormStateInterface $form_state) {
       '#title' => $this->t('Description'),
       '#default_value' => $vocabulary->getDescription(),
     );
+    $form['prevent_duplicates'] = array(
+      '#type' => 'checkbox',
+      '#title' => $this->t('Prevent duplicates'),
+      '#default_value' => $vocabulary->preventDuplicates(),
+    );
 
     // $form['langcode'] is not wrapped in an
     // if ($this->moduleHandler->moduleExists('language')) check because the
diff --git a/core/profiles/standard/config/install/taxonomy.vocabulary.tags.yml b/core/profiles/standard/config/install/taxonomy.vocabulary.tags.yml
index 8fac8f5..b9991f0 100644
--- a/core/profiles/standard/config/install/taxonomy.vocabulary.tags.yml
+++ b/core/profiles/standard/config/install/taxonomy.vocabulary.tags.yml
@@ -6,3 +6,4 @@ vid: tags
 description: 'Use tags to group articles on similar topics into categories.'
 hierarchy: 0
 weight: 0
+prevent_duplicates: false