diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -2457,13 +2457,22 @@
       ->addArgument('slave')
       ->addArgument(NULL)
       ->addArgument('%database.info%');
-    $container->register('cache', 'Drupal\Core\Cache\DatabaseBackend')
-      ->addArgument('cache')
-      ->addArgument(new Reference('database'))
-      ->addTag('cache');
+    // settings.php might have registered an alternative cache implementation
+    // already.
+    if (!$container->has('cache')) {
+      $container->register('cache', 'Drupal\Core\Cache\DatabaseBackend')
+        ->addArgument('cache')
+        ->addArgument(new Reference('database'))
+        ->addTag('cache');
+    }
     foreach (array('bootstrap', 'config', 'page') as $bin) {
-      $definition = clone $container->getDefinition('cache');
-      $container->setDefinition('cache.' . $bin, $definition->replaceArgument(0, $bin));
+      $id = 'cache.' . $bin;
+      if (!$container->has($id)) {
+        // A clone is necessary otherwise the replaceArgument call would change
+        // every definition.
+        $definition = clone $container->getDefinition('cache');
+        $container->setDefinition($id, $definition->replaceArgument(0, $bin));
+      }
     }
 
     $container