diff --git a/core/lib/Drupal/Core/Field/FieldItemBase.php b/core/lib/Drupal/Core/Field/FieldItemBase.php
index 72fa503..e4771b4 100644
--- a/core/lib/Drupal/Core/Field/FieldItemBase.php
+++ b/core/lib/Drupal/Core/Field/FieldItemBase.php
@@ -262,6 +262,8 @@ public static function simplifyAllowedValues(array $structured_values) {
     $values = array();
     foreach ($structured_values as $item) {
       if (is_array($item['label'])) {
+        // Nested elements are embedded in the label.
+        // See structureAllowedValues().
         $item['label'] = static::simplifyAllowedValues($item['label']);
       }
       $values[$item['value']] = $item['label'];
diff --git a/core/modules/options/src/Plugin/Field/FieldType/ListFloatItem.php b/core/modules/options/src/Plugin/Field/FieldType/ListFloatItem.php
index 76da11e..ada4733 100644
--- a/core/modules/options/src/Plugin/Field/FieldType/ListFloatItem.php
+++ b/core/modules/options/src/Plugin/Field/FieldType/ListFloatItem.php
@@ -93,6 +93,8 @@ protected static function validateAllowedValue($option) {
   public static function simplifyAllowedValues(array $structured_values) {
     $values = array();
     foreach ($structured_values as $item) {
+      // Nested elements are embedded in the label.
+      // See FieldItemBase::structureAllowedValues().
       if (is_array($item['label'])) {
         $item['label'] = static::simplifyAllowedValues($item['label']);
       }
diff --git a/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php b/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php
index 186a61d..39d9637 100644
--- a/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php
+++ b/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php
@@ -129,11 +129,17 @@ public static function validateAllowedValues($element, &$form_state) {
     }
     else {
       // Check that keys are valid for the field type.
+      $keys_in_use = array();
       foreach ($values as $item) {
         if ($error = static::validateAllowedValue($item['value'])) {
           \Drupal::formBuilder()->setError($element, $form_state, $error);
           break;
         }
+        if (in_array($item['value'], $keys_in_use)) {
+          \Drupal::formBuilder()->setError($element, $form_state, t('One key can only be used once.'));
+          break;
+        }
+        $keys_in_use[] = $item['value'];
       }
 
       // Prevent removing values currently in use.
diff --git a/core/modules/options/src/Tests/OptionsFieldUITest.php b/core/modules/options/src/Tests/OptionsFieldUITest.php
index 0712329..bf01335 100644
--- a/core/modules/options/src/Tests/OptionsFieldUITest.php
+++ b/core/modules/options/src/Tests/OptionsFieldUITest.php
@@ -118,6 +118,10 @@ function testOptionsAllowedValuesInteger() {
       array('value' => 0, 'label' => 'Zero'),
     );
     $this->assertAllowedValuesInput($string, $array, 'Values not in use can be removed.');
+
+    // Check that the same key can only be used once.
+    $string = "0|Zero\n0|One";
+    $this->assertAllowedValuesInput($string, 'One key can only be used once.', 'Same value cannot be used multiple times.');
   }
 
   /**
@@ -187,6 +191,14 @@ function testOptionsAllowedValuesFloat() {
       array('value' => (float) 0, 'label' => 'Zero'),
     );
     $this->assertAllowedValuesInput($string, $array, 'Values not in use can be removed.');
+
+    // Check that the same key can only be used once.
+    $string = "0.5|Point five\n0.5|Half";
+    $this->assertAllowedValuesInput($string, 'One key can only be used once.', 'Same value cannot be used multiple times.');
+
+    // Check that different forms of the same float value cannot be used.
+    $string = "0|Zero\n.5|Point five\n0.5|Half";
+    $this->assertAllowedValuesInput($string, 'One key can only be used once.', 'Different forms of the same value cannot be used.');
   }
 
   /**
@@ -266,6 +278,19 @@ function testOptionsAllowedValuesText() {
       array('value' => 'Zero', 'label' => 'Zero'),
     );
     $this->assertAllowedValuesInput($string, $array, 'Values not in use can be removed.');
+
+    // Check that string values with dots can be used.
+    $node->delete();
+    $string = "Zero\nexample.com|Example";
+    $array = array(
+      array('value' => 'Zero', 'label' => 'Zero'),
+      array('value' => 'example.com', 'label' => 'Example'),
+    );
+    $this->assertAllowedValuesInput($string, $array, 'String value with dot is supported.');
+
+    // Check that the same key can only be used once.
+    $string = "Zero\nZero";
+    $this->assertAllowedValuesInput($string, 'One key can only be used once.', 'Same value cannot be used multiple times.');
   }
 
   /**
@@ -313,6 +338,10 @@ function testOptionsTrimmedValuesText() {
       array('value' => 'one', 'label' => 'One'),
     );
     $this->assertAllowedValuesInput($string, $array, 'Explicit keys are accepted and trimmed.');
+
+    // Check that the same key can only be used once.
+    $string = " Zero  \n   Zero  ";
+    $this->assertAllowedValuesInput($string, 'One key can only be used once.', 'Same value cannot be used multiple times.');
   }
 
   /**