diff --git a/NEWS b/NEWS
index 5fa80a5..fea60f9 100644
--- a/NEWS
+++ b/NEWS
@@ -13,6 +13,11 @@ Improvements
 * ``AnyMatch`` is now exported properly in ``testtools.matchers``.
   (Robert Collins, Rob Kennedy, github #44)
 
+* In Python 3.3, if there are duplicate test ids, tests.sort() will
+  fail and raise TypeError. Detect the duplicate test ids firstly in
+  sorted_tests() to ensure that all test ids are unique.
+  (Kui Shi, #1243922)
+
 * ``json_content`` is now in the ``__all__`` attribute for
   ``testtools.content``. (Robert Collins)
 
diff --git a/doc/for-framework-folk.rst b/doc/for-framework-folk.rst
index e162859..d105b4f 100644
--- a/doc/for-framework-folk.rst
+++ b/doc/for-framework-folk.rst
@@ -429,6 +429,8 @@ sort_tests, which can be used by non-standard TestSuites to know when they
 should sort their tests. An example implementation can be seen at
 ``FixtureSuite.sorted_tests``.
 
+If there are duplicate test ids in a suite, ValueError will be raised.
+
 filter_by_ids
 -------------
 
diff --git a/testtools/tests/test_testsuite.py b/testtools/tests/test_testsuite.py
index 578bda3..e2c3306 100644
--- a/testtools/tests/test_testsuite.py
+++ b/testtools/tests/test_testsuite.py
@@ -224,6 +224,19 @@ class TestFixtureSuite(TestCase):
         suite.run(LoggingResult([]))
         self.assertEqual(['setUp', 1, 2, 'tearDown'], log)
 
+    def test_fixture_suite_sort(self):
+        log = []
+        class Sample(TestCase):
+            def test_one(self):
+                log.append(1)
+            def test_two(self):
+                log.append(2)
+        fixture = FunctionFixture(
+            lambda: log.append('setUp'),
+            lambda fixture: log.append('tearDown'))
+        suite = FixtureSuite(fixture, [Sample('test_one'), Sample('test_one')])
+        self.assertRaises(ValueError, suite.sort_tests)
+
 
 class TestSortedTests(TestCase):
 
@@ -253,6 +266,13 @@ class TestSortedTests(TestCase):
         suite = sorted_tests(unittest.TestSuite([b, a]))
         self.assertEqual([a, b], list(iterate_tests(suite)))
 
+    def test_duplicate_simple_suites(self):
+        a = PlaceHolder('a')
+        b = PlaceHolder('b')
+        c = PlaceHolder('a')
+        self.assertRaises(
+            ValueError, sorted_tests, unittest.TestSuite([a, b, c]))
+
 
 def test_suite():
     from unittest import TestLoader
diff --git a/testtools/testsuite.py b/testtools/testsuite.py
index 04dd968..9e92e0c 100644
--- a/testtools/testsuite.py
+++ b/testtools/testsuite.py
@@ -303,6 +303,15 @@ def filter_by_ids(suite_or_case, test_ids):
 
 def sorted_tests(suite_or_case, unpack_outer=False):
     """Sort suite_or_case while preserving non-vanilla TestSuites."""
+    # Duplicate test id can induce TypeError in Python 3.3.
+    # Detect the duplicate test id, raise exception when found.
+    seen = set()
+    for test_case in iterate_tests(suite_or_case):
+        test_id = test_case.id()
+        if test_id not in seen:
+            seen.add(test_id)
+        else:
+            raise ValueError('Duplicate test id detected: %s' % (test_id,))
     tests = _flatten_tests(suite_or_case, unpack_outer=unpack_outer)
     tests.sort()
     return unittest.TestSuite([test for (sort_key, test) in tests])