diff -u b/core/modules/user/tests/src/FunctionalJavascript/RegistrationWithUserFieldsTest.php b/core/modules/user/tests/src/FunctionalJavascript/RegistrationWithUserFieldsTest.php
--- b/core/modules/user/tests/src/FunctionalJavascript/RegistrationWithUserFieldsTest.php
+++ b/core/modules/user/tests/src/FunctionalJavascript/RegistrationWithUserFieldsTest.php
@@ -109,29 +109,32 @@
     $field_storage->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED);
     $field_storage->save();
     foreach (['js', 'nojs'] as $js) {
+      $name = $this->randomMachineName();
       $this->drupalGet('user/register');
       $this->assertRegistrationFormCacheTagsWithUserFields();
+      $this->page->fillField('edit-name', $name);
+      $this->page->fillField('edit-mail', $name . '@example.com');
+      $this->page->fillField('test_user_field[0][value]', $value);
       // Add two inputs.
       if ($js == 'js') {
         $field_test_user_add_more = $this->page->find('css', 'input[name="test_user_field_add_more"]');
         $field_test_user_add_more->click();
         $this->webAssert->assertWaitOnAjaxRequest();
+        $this->page->fillField('test_user_field[1][value]', $value . '1');
         $field_test_user_add_more->click();
         $this->webAssert->assertWaitOnAjaxRequest();
+        $this->page->fillField('test_user_field[2][value]', $value . '2');
       }
       else {
         $this->page->pressButton('test_user_field_add_more');
         $this->webAssert->assertWaitOnAjaxRequest();
+        $this->page->fillField('test_user_field[1][value]', $value . '1');
         $this->page->pressButton('test_user_field_add_more');
         $this->webAssert->assertWaitOnAjaxRequest();
+        $this->page->fillField('test_user_field[2][value]', $value . '2');
       }
 
       // Submit with three values.
-      $this->page->fillField('edit-name', $name);
-      $this->page->fillField('edit-mail', $name . '@example.com');
-      $this->page->fillField('test_user_field[0][value]', $value);
-      $this->page->fillField('test_user_field[1][value]', $value . '1');
-      $this->page->fillField('test_user_field[2][value]', $value . '2');
       $this->page->pressButton('edit-submit');
 
       // Check user fields.