diff --git a/tests/handlers/test_message.py b/tests/handlers/test_message.py
new file mode 100644
index 0000000000..64e28bc639
--- /dev/null
+++ b/tests/handlers/test_message.py
@@ -0,0 +1,157 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+from typing import Tuple
+
+from synapse.api.constants import EventTypes
+from synapse.events import EventBase
+from synapse.events.snapshot import EventContext
+from synapse.rest import admin
+from synapse.rest.client.v1 import login, room
+from synapse.types import create_requester
+from synapse.util.stringutils import random_string
+
+from tests import unittest
+
+logger = logging.getLogger(__name__)
+
+
+class EventCreationTestCase(unittest.HomeserverTestCase):
+ servlets = [
+ admin.register_servlets,
+ login.register_servlets,
+ room.register_servlets,
+ ]
+
+ def prepare(self, reactor, clock, hs):
+ self.handler = self.hs.get_event_creation_handler()
+ self.persist_event_storage = self.hs.get_storage().persistence
+
+ self.user_id = self.register_user("tester", "foobar")
+ self.access_token = self.login("tester", "foobar")
+ self.room_id = self.helper.create_room_as(self.user_id, tok=self.access_token)
+
+ self.info = self.get_success(
+ self.hs.get_datastore().get_user_by_access_token(self.access_token,)
+ )
+ self.token_id = self.info["token_id"]
+
+ self.requester = create_requester(self.user_id, access_token_id=self.token_id)
+
+ def _create_duplicate_event(self, txn_id: str) -> Tuple[EventBase, EventContext]:
+ """Create a new event with the given transaction ID. All events produced
+ by this method will be considered duplicates.
+ """
+
+ # We create a new event with a random body, as otherwise we'll produce
+ # *exactly* the same event with the same hash, and so same event ID.
+ return self.get_success(
+ self.handler.create_event(
+ self.requester,
+ {
+ "type": EventTypes.Message,
+ "room_id": self.room_id,
+ "sender": self.requester.user.to_string(),
+ "content": {"msgtype": "m.text", "body": random_string(5)},
+ },
+ token_id=self.token_id,
+ txn_id=txn_id,
+ )
+ )
+
+ def test_duplicated_txn_id(self):
+ """Test that attempting to handle/persist an event with a transaction ID
+ that has already been persisted correctly returns the old event and does
+ *not* produce duplicate messages.
+ """
+
+ txn_id = "something_suitably_random"
+
+ event1, context = self._create_duplicate_event(txn_id)
+
+ ret_event1 = self.get_success(
+ self.handler.handle_new_client_event(self.requester, event1, context)
+ )
+ stream_id1 = ret_event1.internal_metadata.stream_ordering
+
+ self.assertEqual(event1.event_id, ret_event1.event_id)
+
+ event2, context = self._create_duplicate_event(txn_id)
+
+ # We want to test that the deduplication at the persit event end works,
+ # so we want to make sure we test with different events.
+ self.assertNotEqual(event1.event_id, event2.event_id)
+
+ ret_event2 = self.get_success(
+ self.handler.handle_new_client_event(self.requester, event2, context)
+ )
+ stream_id2 = ret_event2.internal_metadata.stream_ordering
+
+ # Assert that the returned values match those from the initial event
+ # rather than the new one.
+ self.assertEqual(ret_event1.event_id, ret_event2.event_id)
+ self.assertEqual(stream_id1, stream_id2)
+
+ # Let's test that calling `persist_event` directly also does the right
+ # thing.
+ event3, context = self._create_duplicate_event(txn_id)
+ self.assertNotEqual(event1.event_id, event3.event_id)
+
+ ret_event3, event_pos3, _ = self.get_success(
+ self.persist_event_storage.persist_event(event3, context)
+ )
+
+ # Assert that the returned values match those from the initial event
+ # rather than the new one.
+ self.assertEqual(ret_event1.event_id, ret_event3.event_id)
+ self.assertEqual(stream_id1, event_pos3.stream)
+
+ # Let's test that calling `persist_events` directly also does the right
+ # thing.
+ event4, context = self._create_duplicate_event(txn_id)
+ self.assertNotEqual(event1.event_id, event3.event_id)
+
+ events, _ = self.get_success(
+ self.persist_event_storage.persist_events([(event3, context)])
+ )
+ ret_event4 = events[0]
+
+ # Assert that the returned values match those from the initial event
+ # rather than the new one.
+ self.assertEqual(ret_event1.event_id, ret_event4.event_id)
+
+ def test_duplicated_txn_id_one_call(self):
+ """Test that we correctly handle duplicates that we try and persist at
+ the same time.
+ """
+
+ txn_id = "something_else_suitably_random"
+
+ # Create two duplicate events to persist at the same time
+ event1, context1 = self._create_duplicate_event(txn_id)
+ event2, context2 = self._create_duplicate_event(txn_id)
+
+ # Ensure their event IDs are different to start with
+ self.assertNotEqual(event1.event_id, event2.event_id)
+
+ events, _ = self.get_success(
+ self.persist_event_storage.persist_events(
+ [(event1, context1), (event2, context2)]
+ )
+ )
+
+ # Check that we've deduplicated the events.
+ self.assertEqual(len(events), 2)
+ self.assertEqual(events[0].event_id, events[1].event_id)
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