diff --git a/config/schema/entity_reference_validators.schema.yml b/config/schema/entity_reference_validators.schema.yml
index 7b28556..1e0938b 100644
--- a/config/schema/entity_reference_validators.schema.yml
+++ b/config/schema/entity_reference_validators.schema.yml
@@ -5,3 +5,6 @@ field.field.*.*.*.third_party.entity_reference_validators:
     circular_reference:
       type: boolean
       label: 'Prevent entity from referencing itself'
+    duplicate_reference:
+      type: boolean
+      label: 'Prevent multiple references to the same entity'
diff --git a/entity_reference_validators.module b/entity_reference_validators.module
index f7dc7a9..2dfaa1c 100644
--- a/entity_reference_validators.module
+++ b/entity_reference_validators.module
@@ -17,6 +17,9 @@ function entity_reference_validators_entity_bundle_field_info_alter(&$fields, \D
     if ($field instanceof ThirdPartySettingsInterface && $field->getThirdPartySetting('entity_reference_validators', 'circular_reference', FALSE)) {
       $field->addConstraint('CircularReference');
     }
+    if ($field instanceof ThirdPartySettingsInterface && $field->getThirdPartySetting('entity_reference_validators', 'duplicate_reference', FALSE)) {
+      $field->addConstraint('DuplicateReference');
+    }
   }
 }
 
@@ -27,21 +30,30 @@ function entity_reference_validators_form_field_config_edit_form_alter(array &$f
   /** @var \Drupal\field\Entity\FieldConfig $field */
   $field = $form_state->getFormObject()->getEntity();
 
-  if ($field->getType() === 'entity_reference' && $field instanceof ThirdPartySettingsInterface
-    // Only add circular reference when the target type is the same as the
-    // field's.
-    && $field->getTargetEntityTypeId() === $field->getItemDefinition()->getSetting('target_type')
-  ) {
+  if ($field->getType() === 'entity_reference' && $field instanceof ThirdPartySettingsInterface) {
+    // Add a fieldset for the validators.
     $form['third_party_settings']['entity_reference_validators']['container'] = [
       '#type' => 'details',
       '#title' => t('Reference validators'),
       '#open' => TRUE,
     ];
-    $form['third_party_settings']['entity_reference_validators']['container']['circular_reference'] = [
+
+    // Only check for circular references when the target type is the same as
+    // the field's.
+    if ($field->getTargetEntityTypeId() === $field->getItemDefinition()->getSetting('target_type')) {
+      $form['third_party_settings']['entity_reference_validators']['container']['circular_reference'] = [
+        '#type' => 'checkbox',
+        '#title' => t('Prevent entity from referencing itself'),
+        '#default_value' => $field->getThirdPartySetting('entity_reference_validators', 'circular_reference', FALSE),
+        '#parents' => ['third_party_settings', 'entity_reference_validators', 'circular_reference'],
+      ];
+    }
+
+    $form['third_party_settings']['entity_reference_validators']['container']['duplicate_reference'] = [
       '#type' => 'checkbox',
-      '#title' => t('Prevent entity from referencing itself'),
-      '#default_value' => $field->getThirdPartySetting('entity_reference_validators', 'circular_reference', FALSE),
-      '#parents' => ['third_party_settings', 'entity_reference_validators', 'circular_reference'],
+      '#title' => t('Prevent entity from referencing duplicates'),
+      '#default_value' => $field->getThirdPartySetting('entity_reference_validators', 'duplicate_reference', FALSE),
+      '#parents' => ['third_party_settings', 'entity_reference_validators', 'duplicate_reference'],
     ];
   }
 }
diff --git a/src/Plugin/Validation/Constraint/DuplicateReferenceConstraint.php b/src/Plugin/Validation/Constraint/DuplicateReferenceConstraint.php
new file mode 100644
index 0000000..e224f81
--- /dev/null
+++ b/src/Plugin/Validation/Constraint/DuplicateReferenceConstraint.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace Drupal\entity_reference_validators\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Entity Reference duplicate reference constraint.
+ *
+ * Verifies that referenced entities do not duplicate a previous reference.
+ *
+ * @Constraint(
+ *   id = "DuplicateReference",
+ *   label = @Translation("Entity Reference duplicate reference", context = "Validation")
+ * )
+ */
+class DuplicateReferenceConstraint extends Constraint {
+
+  /**
+   * The default violation message.
+   *
+   * @var string
+   */
+  public $message = 'The value %label has been entered multiple times.';
+
+}
diff --git a/src/Plugin/Validation/Constraint/DuplicateReferenceConstraintValidator.php b/src/Plugin/Validation/Constraint/DuplicateReferenceConstraintValidator.php
new file mode 100644
index 0000000..a04040f
--- /dev/null
+++ b/src/Plugin/Validation/Constraint/DuplicateReferenceConstraintValidator.php
@@ -0,0 +1,39 @@
+<?php
+
+namespace Drupal\entity_reference_validators\Plugin\Validation\Constraint;
+
+use Drupal\Core\Entity\Element\EntityAutocomplete;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Checks that the same entity is not referenced multiple times.
+ */
+class DuplicateReferenceConstraintValidator extends ConstraintValidator {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($value, Constraint $constraint) {
+    $values = array_map(function ($item) {
+      return $item['target_id'];
+    }, $value->filterEmptyItems()->getValue());
+
+    $values = array_filter($values);
+    $occurrences = array_count_values($values);
+
+    $duplicate_keys = array_keys(array_filter($values, function ($target_id) use ($occurrences) {
+      return $occurrences[$target_id] > 1;
+    }));
+
+    foreach ($duplicate_keys as $key) {
+      $entity = $value->get($key)->entity;
+      $this->context->buildViolation($constraint->message)
+        ->setParameter('%label', EntityAutocomplete::getEntityLabels([$entity]))
+        ->setInvalidValue($entity)
+        ->atPath((string) $key . '.target_id')
+        ->addViolation();
+    }
+  }
+
+}