diff -r b730baee0877 Doc/library/email.errors.rst
--- a/Doc/library/email.errors.rst	Tue May 24 16:39:23 2016 -0700
+++ b/Doc/library/email.errors.rst	Thu May 26 16:19:36 2016 +0200
@@ -73,6 +73,9 @@
 * :class:`NoBoundaryInMultipartDefect` -- A message claimed to be a multipart,
   but had no :mimetype:`boundary` parameter.
 
+* :class:`NotUniqueBoundaryInMultipartDefect` -- A nested multipart message is
+  using the same :mimetype:`boundary` for both an outer and an inner entity.
+
 * :class:`StartBoundaryNotFoundDefect` -- The start boundary claimed in the
   :mailheader:`Content-Type` header was never found.
 
diff -r b730baee0877 Lib/email/errors.py
--- a/Lib/email/errors.py	Tue May 24 16:39:23 2016 -0700
+++ b/Lib/email/errors.py	Thu May 26 16:19:36 2016 +0200
@@ -41,6 +41,9 @@
 class NoBoundaryInMultipartDefect(MessageDefect):
     """A message claimed to be a multipart but had no boundary parameter."""
 
+class NotUniqueBoundaryInMultipartDefect(MessageDefect):
+    """A nested multipart entity is using an outer entity boundary."""
+
 class StartBoundaryNotFoundDefect(MessageDefect):
     """The claimed start boundary was never found."""
 
diff -r b730baee0877 Lib/email/feedparser.py
--- a/Lib/email/feedparser.py	Tue May 24 16:39:23 2016 -0700
+++ b/Lib/email/feedparser.py	Thu May 26 16:19:36 2016 +0200
@@ -318,6 +318,16 @@
                     lines.append(line)
                 self._cur.set_payload(EMPTYSTRING.join(lines))
                 return
+            # The mail composing agent should choose a unique boundary
+            # parameter value that does not contain the boundary parameter
+            # value of an enclosing multipart as a prefix.
+            boundary_reuse = False
+            for m in self._msgstack[:-1]:
+                if m.is_multipart():
+                    enclosing_boundary = m.get_boundary()
+                    if enclosing_boundary is not None:
+                        if boundary.startswith(enclosing_boundary):
+                            boundary_reuse = True
             # Make sure a valid content type was specified per RFC 2045:6.4.
             if (self._cur.get('content-transfer-encoding', '8bit').lower()
                     not in ('7bit', '8bit', 'binary')):
@@ -418,6 +428,17 @@
             # We've seen either the EOF or the end boundary.  If we're still
             # capturing the preamble, we never saw the start boundary.  Note
             # that as a defect and store the captured text as the payload.
+            if boundary_reuse:
+                defect = errors.NotUniqueBoundaryInMultipartDefect()
+                self.policy.handle_defect(self._cur, defect)
+                self._cur.set_payload(EMPTYSTRING.join(preamble))
+                epilogue = []
+                for line in self._input:
+                    if line is NeedMoreData:
+                        yield NeedMoreData
+                        continue
+                self._cur.epilogue = EMPTYSTRING.join(epilogue)
+                return
             if capturing_preamble:
                 defect = errors.StartBoundaryNotFoundDefect()
                 self.policy.handle_defect(self._cur, defect)
diff -r b730baee0877 Lib/test/test_email/test_defect_handling.py
--- a/Lib/test/test_email/test_defect_handling.py	Tue May 24 16:39:23 2016 -0700
+++ b/Lib/test/test_email/test_defect_handling.py	Thu May 26 16:19:36 2016 +0200
@@ -52,15 +52,14 @@
             --MS_Mac_OE_3071477847_720252_MIME_Part--
 
             """)
-        # XXX better would be to actually detect the duplicate.
-        with self._raise_point(errors.StartBoundaryNotFoundDefect):
+        with self._raise_point(errors.NotUniqueBoundaryInMultipartDefect):
             msg = self._str_msg(source)
         if self.raise_expected: return
         inner = msg.get_payload(0)
         self.assertTrue(hasattr(inner, 'defects'))
         self.assertEqual(len(self.get_defects(inner)), 1)
         self.assertIsInstance(self.get_defects(inner)[0],
-                              errors.StartBoundaryNotFoundDefect)
+                              errors.NotUniqueBoundaryInMultipartDefect)
 
     def test_multipart_no_boundary(self):
         source = textwrap.dedent("""\
diff -r b730baee0877 Lib/test/test_email/test_email.py
--- a/Lib/test/test_email/test_email.py	Tue May 24 16:39:23 2016 -0700
+++ b/Lib/test/test_email/test_email.py	Thu May 26 16:19:36 2016 +0200
@@ -2077,12 +2077,11 @@
     # test_defect_handling
     def test_same_boundary_inner_outer(self):
         msg = self._msgobj('msg_15.txt')
-        # XXX We can probably eventually do better
         inner = msg.get_payload(0)
         self.assertTrue(hasattr(inner, 'defects'))
         self.assertEqual(len(inner.defects), 1)
         self.assertIsInstance(inner.defects[0],
-                              errors.StartBoundaryNotFoundDefect)
+                              errors.NotUniqueBoundaryInMultipartDefect)
 
     # test_defect_handling
     def test_multipart_no_boundary(self):
@@ -3497,7 +3496,7 @@
     def test_bytes_parser_on_exception_does_not_close_file(self):
         with openfile('msg_15.txt', 'rb') as fp:
             bytesParser = email.parser.BytesParser
-            self.assertRaises(email.errors.StartBoundaryNotFoundDefect,
+            self.assertRaises(email.errors.NotUniqueBoundaryInMultipartDefect,
                               bytesParser(policy=email.policy.strict).parse,
                               fp)
             self.assertFalse(fp.closed)
@@ -3510,7 +3509,7 @@
     def test_parser_on_exception_does_not_close_file(self):
         with openfile('msg_15.txt', 'r') as fp:
             parser = email.parser.Parser
-            self.assertRaises(email.errors.StartBoundaryNotFoundDefect,
+            self.assertRaises(email.errors.NotUniqueBoundaryInMultipartDefect,
                               parser(policy=email.policy.strict).parse, fp)
             self.assertFalse(fp.closed)