Fix cross-posting held on more than one list.

Closes #176

Also:

* IMessageStore no longer raises a ValueError if the Message-ID already
  exists in the store; it just returns None.
* The internal handle_message() function no longer takes a `preserve`
  argument, since messages are never removed from the IMessageStore.

2 files changed, 21 insertions, 4 deletions

diff --git a/src/mailman/model/messagestore.py b/src/mailman/model/messagestore.py
index f247f94c2..9d0cfbc76 100644
--- a/src/mailman/model/messagestore.py
+++ b/src/mailman/model/messagestore.py
@@ -56,13 +56,11 @@ class MessageStore:
         message_id = message_ids[0]
         if isinstance(message_id, bytes):
             message_id = message_id.decode('ascii')
-        # Complain if the Message-ID already exists in the storage.
+        # If the Message-ID already exists in the store, don't store it again.
         existing = store.query(Message).filter(
             Message.message_id == message_id).first()
         if existing is not None:
-            raise ValueError(
-                'Message ID already exists in message store: {0}'.format(
-                    message_id))
+            return None
         hash32 = add_message_hash(message)
         # Calculate the path on disk where we're going to store this message
         # object, in pickled format.
diff --git a/src/mailman/model/tests/test_messagestore.py b/src/mailman/model/tests/test_messagestore.py
index 6e3142536..998fa36c0 100644
--- a/src/mailman/model/tests/test_messagestore.py
+++ b/src/mailman/model/tests/test_messagestore.py
@@ -137,3 +137,22 @@ X-Message-ID-Hash: abc
         self.assertEqual(len(x_message_id_hashes), 1)
         self.assertEqual(x_message_id_hashes[0],
                          'MS6QLWERIJLGCRF44J7USBFDELMNT2BW')
+
+    def test_add_message_duplicate_okay(self):
+        msg = mfs("""\
+Subject: Once
+Message-ID: <ant>
+
+""")
+        hash32 = self._store.add(msg)
+        stored_msg = self._store.get_message_by_id('<ant>')
+        self.assertEqual(msg['subject'], stored_msg['subject'])
+        self.assertEqual(msg['message-id-hash'], hash32)
+        # A second insertion, even if headers change, does not store the
+        # message twice.
+        del msg['subject']
+        msg['Subject'] = 'Twice'
+        hash32 = self._store.add(msg)
+        stored_msg = self._store.get_message_by_id('<ant>')
+        self.assertNotEqual(msg['subject'], stored_msg['subject'])
+        self.assertIsNone(hash32)