diff --git a/core/includes/module.inc b/core/includes/module.inc
index 287b818..0ea8e7e 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -480,25 +480,19 @@ function module_enable($module_list, $enable_dependencies = TRUE) {
   $disabled_config = config('system.module.disabled');
   foreach ($module_list as $module) {
     // Only process modules that are not already enabled.
-    $enabled = TRUE;
-    $weight = 0;
-    if (!$schema_store->get($module)) {
-      $enabled = FALSE;
-    }
-    elseif (!$module_config->get("enabled.$module")) {
-      $enabled = FALSE;
-      $weight = $disabled_config->get($module);
-    }
+    $enabled = $module_config->get("enabled.$module") !== NULL;
     if (!$enabled) {
+      $weight = $disabled_config->get($module);
+      if ($weight === NULL) {
+        $weight = 0;
+      }
       $module_config
         ->set("enabled.$module", $weight)
         ->set('enabled', module_config_sort($module_config->get('enabled')))
         ->save();
-      if ($weight) {
-        $disabled_config
-          ->clear($module)
-          ->save();
-      }
+      $disabled_config
+        ->clear($module)
+        ->save();
       // Load the module's code.
       drupal_load('module', $module);
       module_load_install($module);
@@ -623,7 +617,6 @@ function module_disable($module_list, $disable_dependents = TRUE) {
         ->save();
       $module_config
         ->clear("enabled.$module")
-        ->set('enabled', module_config_sort($module_config->get('enabled')))
         ->save();
       $invoke_modules[] = $module;
       watchdog('system', '%module module disabled.', array('%module' => $module), WATCHDOG_INFO);
diff --git a/core/modules/system/lib/Drupal/system/Tests/Module/ModuleEnable.php b/core/modules/system/lib/Drupal/system/Tests/Module/ModuleEnable.php
new file mode 100644
index 0000000..b2d08fc
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Module/ModuleEnable.php
@@ -0,0 +1,47 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\system\Tests\Module\ModuleEnable.
+ */
+
+namespace Drupal\system\Tests\Module;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests module_enable().
+ */
+class ModuleEnable extends WebTestBase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Module enable',
+      'description' => 'Tests module_enable().',
+      'group' => 'Module',
+    );
+  }
+
+  /**
+   * Tests enabling User module once more.
+   *
+   * Regression: The installer might enable a module twice due to automatic
+   * dependency resolution. A bug caused the stored weight for User module to
+   * be an array.
+   */
+  function testEnableUserTwice() {
+    module_enable(array('user'), FALSE);
+    $this->assertIdentical(config('system.module')->get('enabled.user'), '0');
+  }
+
+  /**
+   * Tests recorded schema versions of early installed modules in the installer.
+   */
+  function testRequiredModuleSchemaVersions() {
+    $version = drupal_get_installed_schema_version('system', TRUE);
+    $this->assertTrue($version > 0, 'System module version is > 0.');
+    $version = drupal_get_installed_schema_version('user', TRUE);
+    $this->assertTrue($version > 0, 'User module version is > 0.');
+  }
+
+}