bpo-33128 Fix duplicated call to importlib._install_external_importers (GH-6273)

External importers were being added in both phases of the import
system initialisation.

They're only supposed to be added in the second phase, after the
import machinery has been appropriately configured.

Lib/test/test_sys.py

@@ -852,6 +852,9 @@ class SysModuleTest(unittest.TestCase):
         check(-1<<1000, [traceback[-1]])
         check(None, traceback)
 
+    def test_no_duplicates_in_meta_path(self):
+        self.assertEqual(len(sys.meta_path), len(set(sys.meta_path)))
+
 
 @test.support.cpython_only
 class SizeofTest(unittest.TestCase):