diff -r ec00f8570c55 Lib/test/test_imp.py
--- a/Lib/test/test_imp.py	Mon Nov 05 22:23:16 2012 +0200
+++ b/Lib/test/test_imp.py	Tue Nov 06 16:52:39 2012 +0100
@@ -217,6 +217,19 @@
         mod = imp.load_module(example, *x)
         self.assertEqual(mod.__name__, example)
 
+    def test_issue16421_multiple_modules_in_one_dll(self):
+        # Issue 16421: loading several modules from the same compiled file fails
+        m = '_testimportmultiple'
+        f = imp.find_module(m)[1]
+        mod0 = imp.load_dynamic(m, f)
+        mod1 = imp.load_dynamic('foo', f)
+        mod2 = imp.load_dynamic('bar', f)
+        self.assertEqual(mod0.__name__, m)
+        self.assertEqual(mod1.__name__, 'foo')
+        self.assertEqual(mod2.__name__, 'bar')
+        with self.assertRaises(ImportError):
+            imp.load_dynamic('nonexistent', f)
+
     def test_load_dynamic_ImportError_path(self):
         # Issue #1559549 added `name` and `path` attributes to ImportError
         # in order to provide better detail. Issue #10854 implemented those
diff -r ec00f8570c55 Modules/_testimportmultiple.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Modules/_testimportmultiple.c	Tue Nov 06 16:52:39 2012 +0100
@@ -0,0 +1,60 @@
+#include<Python.h>
+
+/* This file defines 3 modules, only the first one is called the same as compiled file:
+
+   * _testimportmodule
+   * foo
+   * bar
+
+*/
+
+static struct PyModuleDef _testimportmultiple = {
+   PyModuleDef_HEAD_INIT,
+   "_testimportmultiple",
+   "_testimportmultiple doc",
+   -1,
+   NULL,
+   NULL,
+   NULL,
+   NULL,
+   NULL
+};
+
+PyMODINIT_FUNC PyInit__testimportmultiple()
+{
+   return PyModule_Create(&_testimportmultiple);
+}
+
+static struct PyModuleDef _foomodule = {
+   PyModuleDef_HEAD_INIT,
+   "foo",
+   "foo doc",
+   -1,
+   NULL,
+   NULL,
+   NULL,
+   NULL,
+   NULL
+};
+
+PyMODINIT_FUNC PyInit_foo()
+{
+   return PyModule_Create(&_foomodule);
+}
+
+static struct PyModuleDef _barmodule = {
+   PyModuleDef_HEAD_INIT,
+   "bar",
+   "bar doc",
+   -1,
+   NULL,
+   NULL,
+   NULL,
+   NULL,
+   NULL
+};
+
+PyMODINIT_FUNC PyInit_bar(){
+   return PyModule_Create(&_barmodule);
+}
+
diff -r ec00f8570c55 Python/import.c
--- a/Python/import.c	Mon Nov 05 22:23:16 2012 +0200
+++ b/Python/import.c	Tue Nov 06 16:52:39 2012 +0100
@@ -449,12 +449,12 @@
 
 /* Magic for extension modules (built-in as well as dynamically
    loaded).  To prevent initializing an extension module more than
-   once, we keep a static dictionary 'extensions' keyed by module name
-   (for built-in modules) or by filename (for dynamically loaded
-   modules), containing these modules.  A copy of the module's
-   dictionary is stored by calling _PyImport_FixupExtensionObject()
-   immediately after the module initialization function succeeds.  A
-   copy can be retrieved from there by calling
+   once, we keep a static dictionary 'extensions' keyed by the tuple
+   (module name, module name)  (for built-in modules) or by
+   (filename, module name) (for dynamically loaded modules), containing these
+   modules.  A copy of the module's dictionary is stored by calling
+   _PyImport_FixupExtensionObject() immediately after the module initialization
+   function succeeds.  A copy can be retrieved from there by calling
    _PyImport_FindExtensionObject().
 
    Modules which do support multiple initialization set their m_size
@@ -505,7 +505,7 @@
         if (def->m_base.m_copy == NULL)
             return -1;
     }
-    PyDict_SetItem(extensions, filename, (PyObject*)def);
+    PyDict_SetItem(extensions, PyTuple_Pack(2,filename, name), (PyObject*)def);
     return 0;
 }
 
@@ -529,7 +529,7 @@
     PyModuleDef* def;
     if (extensions == NULL)
         return NULL;
-    def = (PyModuleDef*)PyDict_GetItem(extensions, filename);
+    def = (PyModuleDef*)PyDict_GetItem(extensions, PyTuple_Pack(2, filename, name));
     if (def == NULL)
         return NULL;
     if (def->m_size == -1) {
diff -r ec00f8570c55 setup.py
--- a/setup.py	Mon Nov 05 22:23:16 2012 +0200
+++ b/setup.py	Tue Nov 06 16:52:39 2012 +0100
@@ -591,6 +591,8 @@
                                depends=['testcapi_long.h']) )
         # Python PEP-3118 (buffer protocol) test module
         exts.append( Extension('_testbuffer', ['_testbuffer.c']) )
+        # Test loading multiple modules from one compiled file (http://bugs.python.org/issue16421)
+        exts.append( Extension('_testimportmultiple', ['_testimportmultiple.c']) )
         # profiler (_lsprof is for cProfile.py)
         exts.append( Extension('_lsprof', ['_lsprof.c', 'rotatingtree.c']) )
         # static Unicode character database