diff --git a/core/modules/forum/config/optional/taxonomy.vocabulary.forums.yml b/core/modules/forum/config/optional/taxonomy.vocabulary.forums.yml
index b5c51eb61e..ef2e54afd6 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 abada74b35..dc9f8cc34c 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 efa08e154e..e3c6c82598 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 535fa8c425..a8987afdd7 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 b0d1ac13d2..e3d522c8d6 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",
  *   }
  * )
  */
@@ -88,6 +89,13 @@ class Vocabulary extends ConfigEntityBundleBase implements VocabularyInterface {
    */
   protected $weight = 0;
 
+  /**
+   * Prevents to create terms with the same name.
+   *
+   * @var bool
+   */
+  protected $prevent_duplicates = FALSE;
+
   /**
    * {@inheritdoc}
    */
@@ -117,6 +125,13 @@ public function getDescription() {
     return $this->description;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function preventDuplicates() {
+    return $this->prevent_duplicates;
+  }
+
   /**
    * {@inheritdoc}
    */
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 0000000000..4bf8fbef84
--- /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 $message = 'The term %value already exists in vocabulary %bundle.';
+
+
+}
diff --git a/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraintValidator.php b/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraintValidator.php
new file mode 100644
index 0000000000..a9ed9a2ec8
--- /dev/null
+++ b/core/modules/taxonomy/src/Plugin/Validation/Constraint/TermNameConstraintValidator.php
@@ -0,0 +1,59 @@
+<?php
+
+namespace Drupal\taxonomy\Plugin\Validation\Constraint;
+
+use Drupal\Component\Utility\Unicode;
+use Drupal\taxonomy\Entity\Vocabulary;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Validates the TermName constraint.
+ */
+class TermNameConstraintValidator extends ConstraintValidator {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($items, Constraint $constraint) {
+
+    if (!$item = $items->first()) {
+      return;
+    }
+
+    /** @var \Drupal\Core\Entity\ContentEntityInterface $entity */
+    $entity = $items->getEntity();
+
+    if (!Vocabulary::load($entity->bundle())->preventDuplicates()) {
+      return;
+    }
+
+    $field_definition = $items->getFieldDefinition();
+
+    $entity_type_id = $entity->getEntityTypeId();
+    $entity_type = $entity->getEntityType();
+    $bundle_key = $entity_type->getKey('bundle');
+    $id_key = $entity_type->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) $entity->id(), '<>')
+      ->condition($field_definition->getName(), $item->value)
+      ->condition($bundle_key, $entity->bundle())
+      ->accessCheck(FALSE)
+      ->range(0, 1)
+      ->count()
+      ->execute();
+
+    if ($value_taken) {
+      $this->context->addViolation($constraint->message, [
+        '%value' => $item->value,
+        '%bundle' => $entity->bundle(),
+        '@entity_type' => $entity_type->getLowercaseLabel(),
+        '@field_name' => Unicode::strtolower($field_definition->getLabel()),
+      ]);
+    }
+
+  }
+
+}
diff --git a/core/modules/taxonomy/src/Tests/TermTest.php b/core/modules/taxonomy/src/Tests/TermTest.php
index 42f92b1b4b..ff529ea424 100644
--- a/core/modules/taxonomy/src/Tests/TermTest.php
+++ b/core/modules/taxonomy/src/Tests/TermTest.php
@@ -5,6 +5,7 @@
 use Drupal\Component\Utility\Tags;
 use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Language\LanguageInterface;
 use Drupal\field\Entity\FieldConfig;
 use Drupal\taxonomy\Entity\Term;
 use Drupal\taxonomy\Entity\Vocabulary;
@@ -585,4 +586,59 @@ public function testTermBreadcrumbs() {
     $this->assertEscaped((string) $breadcrumbs[1], 'breadcrumbs displayed and escaped.');
   }
 
+  /**
+   * Check that it's not possible to add a term with same name multiple times.
+   */
+  public function testTermUniqueName() {
+
+    $vocabulary = Vocabulary::create([
+      'name' => $this->randomMachineName(),
+      'description' => $this->randomMachineName(),
+      'vid' => Unicode::strtolower($this->randomMachineName()),
+      'langcode' => LanguageInterface::LANGCODE_NOT_SPECIFIED,
+      'weight' => mt_rand(0, 10),
+      'prevent_duplicates' => TRUE,
+    ]);
+    $vocabulary->save();
+
+    $termName = $this->randomMachineName(14);
+    $edit = [
+      'name[0][value]' => $termName,
+      'description[0][value]' => $this->randomMachineName(100),
+      'parent[]' => [0],
+    ];
+
+    // Create the term.
+    $this->drupalPostForm('admin/structure/taxonomy/manage/' . $vocabulary->id() . '/add', $edit, t('Save'));
+
+    // Check if term was created.
+    $terms = taxonomy_term_load_multiple_by_name($edit['name[0][value]']);
+    /** @var \Drupal\taxonomy\Entity\Term $term */
+    $term = reset($terms);
+    $this->assertNotNull($term, 'Term found in database.');
+    $this->assertText('Created new term ' . $term->label());
+
+    // Try to create a term with the same name.
+    $this->drupalPostForm('admin/structure/taxonomy/manage/' . $vocabulary->id() . '/add', $edit, t('Save'));
+
+    $this->assertText('The term ' . $term->label() . ' already exists in vocabulary ' . $term->bundle());
+
+    // Create a new term name.
+    $edit = [
+      'name[0][value]' => $edit['name[0][value]'] . '_2',
+      'description[0][value]' => $this->randomMachineName(100),
+      'parent[]' => [0],
+    ];
+
+    // Create the term.
+    $this->drupalPostForm('admin/structure/taxonomy/manage/' . $vocabulary->id() . '/add', $edit, t('Save'));
+
+    // Check if term was created.
+    $terms = taxonomy_term_load_multiple_by_name($edit['name[0][value]']);
+    /** @var \Drupal\taxonomy\Entity\Term $term */
+    $term = reset($terms);
+    $this->assertNotNull($term, 'Term found in database.');
+    $this->assertText('Created new term ' . $term->label());
+  }
+
 }
diff --git a/core/modules/taxonomy/src/VocabularyForm.php b/core/modules/taxonomy/src/VocabularyForm.php
index 63f0ccb8ab..cf388377fa 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'] = [
+      '#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/modules/taxonomy/src/VocabularyInterface.php b/core/modules/taxonomy/src/VocabularyInterface.php
index cb54e20184..6a7e2c4044 100644
--- a/core/modules/taxonomy/src/VocabularyInterface.php
+++ b/core/modules/taxonomy/src/VocabularyInterface.php
@@ -54,4 +54,12 @@ public function setHierarchy($hierarchy);
    */
   public function getDescription();
 
+  /**
+   * Indicates if multiple terms with same name are forbidden in the vocabulary.
+   *
+   * @return bool
+   *   TRUE if same name is forbidden, FALSE if allowed.
+   */
+  public function preventDuplicates();
+
 }
diff --git a/core/profiles/standard/config/install/taxonomy.vocabulary.tags.yml b/core/profiles/standard/config/install/taxonomy.vocabulary.tags.yml
index 8fac8f5a2d..b9991f0523 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