2c2
< index 3e67737..6f32c6c 100644
---
> index 3e67737..e7f5377 100644
5,6c5,17
< @@ -339,6 +339,9 @@ protected function sendPlaceholders(array $placeholders, array $placeholder_orde
<        return;
---
> @@ -508,7 +508,9 @@ protected function renderPlaceholder($placeholder, array $placeholder_render_arr
>     *
>     * @return array
>     *   Indexed array; the order in which the BigPipe placeholders must be sent.
> -   *   Values are the BigPipe placeholder IDs.
> +   *   Values are the BigPipe placeholder IDs. Note that only unique
> +   *   placeholders are kept: if the same placeholder occurs multiple times, we
> +   *   only keep the first occurrence.
>     */
>    protected function getPlaceholderOrder($html) {
>      $fragments = explode('<div data-big-pipe-placeholder-id="', $html);
> @@ -521,7 +523,7 @@ protected function getPlaceholderOrder($html) {
>        $order[] = $placeholder;
9,10c20,52
< +    // Keep track of the placeholders already sent.
< +    $sent_placeholders = [];
---
> -    return $order;
> +    return array_unique($order);
>    }
>  
>  }
> diff --git a/core/modules/big_pipe/src/Tests/BigPipeTest.php b/core/modules/big_pipe/src/Tests/BigPipeTest.php
> index f947c46..1fec519 100644
> --- a/core/modules/big_pipe/src/Tests/BigPipeTest.php
> +++ b/core/modules/big_pipe/src/Tests/BigPipeTest.php
> @@ -269,6 +269,27 @@ public function testBigPipeNoJs() {
>      unlink(\Drupal::root() . '/' . $this->siteDirectory . '/error.log');
>    }
>  
> +  /**
> +   * Tests BigPipe responses do not include duplicate placeholder replacements.
> +   */
> +  public function testBigPipeJsMultipleOccurrencePlaceholders() {
> +    $this->drupalLogin($this->rootUser);
> +    $this->assertSessionCookieExists(TRUE);
> +    $this->assertBigPipeNoJsCookieExists(FALSE);
> +
> +    // By not calling performMetaRefresh() here, we simulate JavaScript being
> +    // enabled, because as far as the BigPipe module is concerned, JavaScript is
> +    // enabled in the browser as long as the BigPipe no-JS cookie is *not* set.
> +    // @see setUp()
> +    // @see performMetaRefresh()
> +
> +    $this->drupalGet(Url::fromRoute('big_pipe_duplicate_placeholders'));
> +    $big_pipe_placeholder_id = 'callback=Drupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3AplaceholderContent&amp;args[0]=awesome&amp;token=d13e3d6c';
> +    $expected_placeholder_replacement = '<script type="application/vnd.drupal-ajax" data-big-pipe-replacement-for-placeholder-with-id="' . $big_pipe_placeholder_id . '">';
> +    $raw_content = $this->getRawContent();
> +    $this->assertTrue(substr_count($raw_content, $expected_placeholder_replacement) == 1, 'Only one placeholder replacement was found for the duplicate #lazy_builder arrays.');
> +  }
12,31c54,79
<      // Send the start signal.
<      print "\n";
<      print static::START_SIGNAL;
< @@ -382,7 +385,17 @@ protected function sendPlaceholders(array $placeholders, array $placeholder_orde
<        // placeholder ID (which has HTML entities encoded since we use it to find
<        // the placeholders).
<        $big_pipe_js_placeholder_id = Html::decodeEntities($placeholder_id);
< -      $ajax_response->addCommand(new ReplaceCommand(sprintf('[data-big-pipe-placeholder-id="%s"]', $big_pipe_js_placeholder_id), $elements['#markup']));
< +
< +      // If there are identical placeholders, only add the ReplaceCommand once,
< +      // because all of the placeholders will be replaced the first time the
< +      // ReplaceCommand runs. Otherwise, Drupal.detachBehaviors() and
< +      // Drupal.attachBehaviors() will run on the context of the entire
< +      // document for subsequent ReplaceCommand instances, because all
< +      // placeholders with the identical ID will have been removed from the DOM
< +      // and their context will no longer be available to JavaScript.
< +      if (!in_array($big_pipe_js_placeholder_id, $sent_placeholders)) {
< +        $ajax_response->addCommand(new ReplaceCommand(sprintf('[data-big-pipe-placeholder-id="%s"]', $big_pipe_js_placeholder_id), $elements['#markup']));
< +      }
<        $ajax_response->setAttachments($elements['#attached']);
---
>    protected function assertBigPipeResponseHeadersPresent() {
>      $this->pass('Verifying BigPipe response headers…', 'Debug');
>      $this->assertTrue(FALSE !== strpos($this->drupalGetHeader('Cache-Control'), 'private'), 'Cache-Control header set to "private".');
> diff --git a/core/modules/big_pipe/tests/modules/big_pipe_test/big_pipe_test.routing.yml b/core/modules/big_pipe/tests/modules/big_pipe_test/big_pipe_test.routing.yml
> index 4979406..e60481c 100644
> --- a/core/modules/big_pipe/tests/modules/big_pipe_test/big_pipe_test.routing.yml
> +++ b/core/modules/big_pipe/tests/modules/big_pipe_test/big_pipe_test.routing.yml
> @@ -15,3 +15,12 @@ no_big_pipe:
>      _no_big_pipe: TRUE
>    requirements:
>      _access: 'TRUE'
> +
> +big_pipe_duplicate_placeholders:
> +  path: '/big_pipe_duplicate_placeholders'
> +  defaults:
> +    _controller: '\Drupal\big_pipe_test\BigPipeTestController::duplicates'
> +    _title: 'BigPipe with duplicate placeholders test'
> +  requirements:
> +    _access: 'TRUE'
> +
> diff --git a/core/modules/big_pipe/tests/modules/big_pipe_test/src/BigPipeTestController.php b/core/modules/big_pipe/tests/modules/big_pipe_test/src/BigPipeTestController.php
> index 450a464..1327f37 100644
> --- a/core/modules/big_pipe/tests/modules/big_pipe_test/src/BigPipeTestController.php
> +++ b/core/modules/big_pipe/tests/modules/big_pipe_test/src/BigPipeTestController.php
> @@ -53,6 +53,30 @@ public static function nope() {
>    }
33,36c81,111
<        // Push a fake request with the asset libraries loaded so far and dispatch
< @@ -418,6 +431,9 @@ protected function sendPlaceholders(array $placeholders, array $placeholder_orde
<        print $output;
<        flush();
---
>    /**
> +   * Returns a render array with duplicate lazy_builder placeholders to test
> +   * BigPipe handling of duplicates.
> +   *
> +   * @return array
> +   */
> +  public function duplicates() {
> +    $adjective = 'awesome';
> +    return [
> +      'item1' => [
> +        '#lazy_builder' => [static::class . '::placeholderContent', [$adjective]],
> +        '#create_placeholder' => TRUE,
> +      ],
> +      'item2' => [
> +        '#lazy_builder' => [static::class . '::placeholderContent', [$adjective]],
> +        '#create_placeholder' => TRUE,
> +      ],
> +      'item3' => [
> +        '#lazy_builder' => [static::class . '::placeholderContent', [$adjective]],
> +        '#create_placeholder' => TRUE,
> +      ],
> +    ];
> +  }
> +
> +  /**
>     * #lazy_builder callback; builds <time> markup with current time.
>     *
>     * Note: does not actually use current time, that would complicate testing.
> @@ -98,4 +122,20 @@ public static function responseException() {
>      return ['#plain_text' => BigPipeTestSubscriber::CONTENT_TRIGGER_EXCEPTION];
>    }
38,39c113,127
< +      // Track the IDs of the placeholders already sent.
< +      $sent_placeholders[] = $big_pipe_js_placeholder_id;
---
> +  /**
> +   * #lazy_builder callback; builds markup using a provided adjective.
> +   *
> +   * @param string $adjective
> +   *   The adjective to use in the rendered markup.
> +   *
> +   * @return array
> +   *   The render array.
> +   */
> +  public static function placeholderContent($adjective) {
> +    return [
> +      '#markup' => t('I think llamas are @adjective', ['@adjective' => $adjective]),
> +      '#cache' => ['max-age' => 0]
> +    ];
> +  }
41,44c129,130
<        // Another placeholder was rendered and sent, track the set of asset
<        // libraries sent so far. Any new settings are already sent; we don't need
<        // to track those.
< diff --git a/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipePlaceholderTest.php b/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipePlaceholderTest.php
---
>  }
> diff --git a/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipeRegressionTest.php b/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipeRegressionTest.php
46c132
< index 0000000..1b045e4
---
> index 0000000..f093791
48,49c134,135
< +++ b/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipePlaceholderTest.php
< @@ -0,0 +1,116 @@
---
> +++ b/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipeRegressionTest.php
> @@ -0,0 +1,115 @@
67c153
< + * @group comment
---
> + * @group big_pipe
69c155
< +class BigPipePlaceholderTest extends JavascriptTestBase {
---
> +class BigPipeRegressionTest extends JavascriptTestBase {
97,98d182
< +    // Clear the theme registry.
< +    $this->container->set('theme.registry', NULL);
99a184,189
> +  }
> +
> +  /**
> +   * Ensure comment form works with history and big_pipe modules.
> +   */
> +  public function testCommentFormRegression_2698811() {
135d224
< +  }
137,140d225
< +  /**
< +   * Ensure comment form works with history and big_pipe modules.
< +   */
< +  public function testCommentForm() {