diff --git a/fbl.module b/fbl.module
index 5beae28..78d2b27 100644
--- a/fbl.module
+++ b/fbl.module
@@ -70,6 +70,22 @@ function fbl_form_alter(&$form, FormStateInterface $form_state, $form_id) {
       $form['name']['#description'] = t("@unique_field_desc", array('@unique_field_desc' => $fbl_value['field_desc']));
     }
   }
+  if ($form_id == 'user_form' || $form_id == 'user_register_form') {
+    $form['#validate'][] = 'fbl_user_register_validate';
+  }
+}
+
+function fbl_user_register_validate($form, FormStateInterface &$form_state) {
+  $fbl_config = \Drupal::config('fbl.settings');
+  $fbl_value = $fbl_config->get('field_based_login');
+  $field_name = $fbl_value['field'];
+  $new_user_input = $form_state->getValue($field_name);
+  $query = \Drupal::entityQuery('user');
+  $query->condition($field_name . '.value', $new_user_input[0]['value']);
+  $results = $query->execute();
+  if (count($results) >= 1) {
+    $form_state->setErrorByName($field_name, t('The entered value is already exists. Please enter different value'));
+  }
 }
 
 /*