diff --git a/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php b/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
index 2a9bbb9..278a823 100644
--- a/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
+++ b/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
@@ -36,7 +36,7 @@ public function __construct(array $configuration) {
   /**
    * {@inheritdoc}
    */
-  public function load($name, callable $success = NULL, $wait = 200000, $retry = 5) {
+  public function load($name, callable $success = NULL, $force = FALSE, $wait = 200000, $retry = 5) {
     // The FALSE returned on failure is enough for the caller to handle this,
     // we do not want a warning too. Also, no race conditions in read only
     // storage so ignore all those parameters.
@@ -46,7 +46,7 @@ public function load($name, callable $success = NULL, $wait = 200000, $retry = 5
   /**
    * {@inheritdoc}
    */
-  public function loadClass($name, $class_name, $wait = 200000, $retry = 5) {
+  public function loadClass($name, $class_name, $force = FALSE, $wait = 200000, $retry = 5) {
     // No race conditions in read only storage, huzzah!
     return $this->load($name);
   }
diff --git a/core/lib/Drupal/Component/PhpStorage/FileStorage.php b/core/lib/Drupal/Component/PhpStorage/FileStorage.php
index 87f5c18..bef7a05 100644
--- a/core/lib/Drupal/Component/PhpStorage/FileStorage.php
+++ b/core/lib/Drupal/Component/PhpStorage/FileStorage.php
@@ -35,9 +35,14 @@ public function __construct(array $configuration) {
   /**
    * {@inheritdoc}
    */
-  public function load($name, callable $success = NULL, $wait = 200000, $retry = 5) {
+  public function load($name, callable $success = NULL, $force = FALSE, $wait = 200000, $retry = 5) {
     if (!isset($success)) {
-      return $this->doIncludeOnce($name);
+      // Without a success callback, the only way to be safe is to include the
+      // file once.
+      return $force ? $this->doInclude($name) : $this->doIncludeOnce($name);
+    }
+    if (!$force && $success()) {
+      return TRUE;
     }
     // If the success callback fails after loading the file a race condition is
     // very likely so wait some random time allowing the racing process to
@@ -53,7 +58,7 @@ public function load($name, callable $success = NULL, $wait = 200000, $retry = 5
   /**
    * {@inheritdoc}
    */
-  public function loadClass($name, $class_name = NULL, $wait = 200000, $retry = 5) {
+  public function loadClass($name, $class_name = NULL, $force = FALSE, $wait = 200000, $retry = 5) {
     // The FALSE returned on failure is enough for the caller to handle this,
     // we do not want a warning too.
     return $this->load($name, function () use($class_name) { return class_exists($class_name, FALSE); }, $wait, $retry);
diff --git a/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php b/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
index fadc7ad..587c56d 100644
--- a/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
+++ b/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
@@ -24,12 +24,21 @@
    * This function may be called for a file that doesn't exist, and that should
    * not result in errors.
    *
+   * It is safe to call this function multiple times: the relevant code will
+   * only be loaded once.
+   *
    * @param string $name
    *   The virtual file name. Can be a relative path.
    * @param callable $success
    *   A race condition might cause the file to exist but be empty. If this
    *   parameter is specified the system will retry loading if the callback
-   *   return FALSE to allow the racing process to finish.
+   *   return FALSE to allow the racing process to finish. It is strongly
+   *   recommended to supply this argument or use loadClass() instead to load
+   *   a class. Otherwise, if the same file is written by anotehr process,
+   *   a false positive might occur.
+   * @param bool $force
+   *   Override the safety guarantee and force loading. For example, the caller
+   *   might've already determined the code does not exist before calling.
    * @param int $wait
    *   Wait a random amount of time between $wait and 2 * $wait microseconds
    *   between retries.
@@ -39,21 +48,26 @@
    * @return
    *   TRUE on success, FALSE otherwise.
    */
-  public function load($name, callable $success = NULL, $wait = 200000, $retry = 5);
+  public function load($name, callable $success = NULL, $force = FALSE, $wait = 200000, $retry = 5);
 
   /**
    * Loads a class from storage.
    *
-   * Depending on storage implementation, exists() checks can be expensive, so
-   * this function may be called for a file that doesn't exist, and that should
+   * This function may be called for a file that doesn't exist, and that should
    * not result in errors.
    *
+   * It is safe to call this function multiple times: the class will only be
+   * loaded once.
+   *
    * @param string $name
    *   The virtual file name. Can be a relative path.
    * @param string $class_name
    *   A race condition might cause the file to exist but be empty. The system
    *   will retry loading if the class does not exist to allow the racing
    *   process to finish.
+   * @param bool $force
+   *   Override the safety guarantee and force loading. For example, the caller
+   *   might've already determined the class does not exist before calling.
    * @param int $wait
    *   Wait a random amount of time between $wait and 2 * $wait microseconds
    *   between retries.
@@ -63,7 +77,7 @@ public function load($name, callable $success = NULL, $wait = 200000, $retry = 5
    * @return
    *   TRUE on success, FALSE otherwise.
    */
-  public function loadClass($name, $class_name, $wait = 200000, $retry = 5);
+  public function loadClass($name, $class_name, $force = FALSE, $wait = 200000, $retry = 5);
 
   /**
    * Saves PHP code to storage.
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 755950c..03c6a7f 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -746,9 +746,7 @@ protected function initializeContainer($rebuild = FALSE) {
       $fully_qualified_class_name = '\\' . $this->getClassNamespace() . '\\' . $class_name;
 
       // First, try to load from storage.
-      if (!class_exists($fully_qualified_class_name, FALSE)) {
-        $this->storage()->loadClass($class_name . '.php', $fully_qualified_class_name);
-      }
+      $this->storage()->loadClass($class_name . '.php', $fully_qualified_class_name);
       // If the load succeeded or the class already existed, use it.
       if (class_exists($fully_qualified_class_name, FALSE)) {
         $container = new $fully_qualified_class_name;
diff --git a/core/lib/Drupal/Core/Template/TwigEnvironment.php b/core/lib/Drupal/Core/Template/TwigEnvironment.php
index 3807888..c30ca2e 100644
--- a/core/lib/Drupal/Core/Template/TwigEnvironment.php
+++ b/core/lib/Drupal/Core/Template/TwigEnvironment.php
@@ -127,9 +127,9 @@ public function loadTemplate($name, $index = NULL) {
           $this->updateCompiledTemplate($cache_filename, $name);
         }
 
-        if (!$this->storage()->loadClass($cache_filename, $cls)) {
+        if (!$this->storage()->loadClass($cache_filename, $cls, TRUE)) {
           $this->updateCompiledTemplate($cache_filename, $name);
-          $this->storage()->loadClass($cache_filename, $cls);
+          $this->storage()->loadClass($cache_filename, $cls, TRUE);
         }
       }
     }