Add test and change log entry.

CHANGES.rst

@@ -9,6 +9,8 @@ Changelog
 
 - No longer use icons which got deleted in Zope 4.
 
+- Fix sorting in <dtml-in> for duplicate entries in Python 3.
+
 
 3.0b2 (2017-11-03)
 ------------------

src/DocumentTemplate/tests/test_DT_In.py

@@ -0,0 +1,34 @@
+import unittest
+
+
+class DummySection(object):
+    blocks = ['dummy']
+
+
+class TestIn(unittest.TestCase):
+    """Testing ..DT_in.InClass."""
+
+    def _getTargetClass(self):
+        from DocumentTemplate.DT_In import InClass
+        return InClass
+
+    def _makeOne(self, *args):
+        blocks = [('in', ' '.join(args), DummySection())]
+        return self._getTargetClass()(blocks)
+
+    def test_sort_sequence(self):
+        """It does not break on duplicate sort keys at a list of dicts."""
+        stmt = self._makeOne('seq', 'mapping', 'sort=key')
+        seq = [
+            {'key': 'c', 'data': '3'},
+            {'key': 'a', 'data': '1'},
+            {'key': 'b', 'data': '2'},
+            {'key': 'a', 'data': '2'},
+        ]
+        result = stmt.sort_sequence(seq, 'key')
+        self.assertEqual([
+            {'key': 'a', 'data': '1'},
+            {'key': 'a', 'data': '2'},
+            {'key': 'b', 'data': '2'},
+            {'key': 'c', 'data': '3'},
+        ], result)