diff --git a/src/Composer/InstalledVersions.php b/src/Composer/InstalledVersions.php
index 07b32ed6e..6d29bff66 100644
--- a/src/Composer/InstalledVersions.php
+++ b/src/Composer/InstalledVersions.php
@@ -32,6 +32,11 @@ class InstalledVersions
      */
     private static $installed;
 
+    /**
+     * @var bool
+     */
+    private static $installedIsLocalDir;
+
     /**
      * @var bool|null
      */
@@ -309,6 +314,12 @@ class InstalledVersions
     {
         self::$installed = $data;
         self::$installedByVendor = array();
+
+        // when using reload, we disable the duplicate protection to ensure that self::$installed data is
+        // always returned, but we cannot know whether it comes from the installed.php in __DIR__ or not,
+        // so we have to assume it does not, and that may result in duplicate data being returned when listing
+        // all installed packages for example
+        self::$installedIsLocalDir = false;
     }
 
     /**
@@ -325,7 +336,9 @@ class InstalledVersions
         $copiedLocalDir = false;
 
         if (self::$canGetVendors) {
+            $selfDir = strtr(__DIR__, '\\', '/');
             foreach (ClassLoader::getRegisteredLoaders() as $vendorDir => $loader) {
+                $vendorDir = strtr($vendorDir, '\\', '/');
                 if (isset(self::$installedByVendor[$vendorDir])) {
                     $installed[] = self::$installedByVendor[$vendorDir];
                 } elseif (is_file($vendorDir.'/composer/installed.php')) {
@@ -333,11 +346,14 @@ class InstalledVersions
                     $required = require $vendorDir.'/composer/installed.php';
                     self::$installedByVendor[$vendorDir] = $required;
                     $installed[] = $required;
-                    if (strtr($vendorDir.'/composer', '\\', '/') === strtr(__DIR__, '\\', '/')) {
+                    if (self::$installed === null && $vendorDir.'/composer' === $selfDir) {
                         self::$installed = $required;
-                        $copiedLocalDir = true;
+                        self::$installedIsLocalDir = true;
                     }
                 }
+                if (self::$installedIsLocalDir && $vendorDir.'/composer' === $selfDir) {
+                    $copiedLocalDir = true;
+                }
             }
         }
 
diff --git a/tests/Composer/Test/InstalledVersionsTest.php b/tests/Composer/Test/InstalledVersionsTest.php
index fdcf28fbb..295b16882 100644
--- a/tests/Composer/Test/InstalledVersionsTest.php
+++ b/tests/Composer/Test/InstalledVersionsTest.php
@@ -266,4 +266,31 @@ class InstalledVersionsTest extends TestCase
         self::assertSame('/foo/bar/vendor/c/c', \Composer\InstalledVersions::getInstallPath('c/c'));
         self::assertNull(\Composer\InstalledVersions::getInstallPath('foo/impl'));
     }
+
+    public function testWithClassLoaderLoaded(): void
+    {
+        // disable multiple-ClassLoader-based checks of InstalledVersions by making it seem like no
+        // class loaders are registered
+        $prop = new \ReflectionProperty(ClassLoader::class, 'registeredLoaders');
+        $prop->setAccessible(true);
+        $prop->setValue(null, array_slice(self::$previousRegisteredLoaders, 0, 1, true));
+
+        $prop2 = new \ReflectionProperty(InstalledVersions::class, 'installedIsLocalDir');
+        $prop2->setAccessible(true);
+        $prop2->setValue(null, true);
+
+        self::assertFalse(InstalledVersions::isInstalled('foo/bar'));
+        InstalledVersions::reload([
+            'root' => InstalledVersions::getRootPackage(),
+            'versions' => [
+                'foo/bar' => [
+                    'version' => '1.0.0',
+                    'dev_requirement' => false,
+                ],
+            ],
+        ]);
+        self::assertTrue(InstalledVersions::isInstalled('foo/bar'));
+
+        $prop->setValue(null, []);
+    }
 }
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 05b88f2f6..2213d7421 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -10,7 +10,6 @@
  * file that was distributed with this source code.
  */
 
-use Composer\InstalledVersions;
 use Composer\Util\Platform;
 
 error_reporting(E_ALL);
@@ -20,10 +19,10 @@ if (function_exists('date_default_timezone_set') && function_exists('date_defaul
 }
 
 require __DIR__.'/../src/bootstrap.php';
-
-if (!class_exists(InstalledVersions::class, false)) {
-    require __DIR__.'/../src/Composer/InstalledVersions.php';
-}
+// ensure we always use the latest InstalledVersions.php even if an older composer ran the install, but we need
+// to have it included from vendor dir and not from src/ otherwise some gated check in the code will not work
+copy(__DIR__.'/../src/Composer/InstalledVersions.php', __DIR__.'/../vendor/composer/InstalledVersions.php');
+require __DIR__.'/../vendor/composer/InstalledVersions.php';
 
 Platform::putEnv('COMPOSER_TESTS_ARE_RUNNING', '1');