Add a note about duplicated messages with timescaledb

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

docs/datanommer_datagrepper/pg_timescaledb.rst

@@ -66,6 +66,7 @@ timescaledb uses table partitioning as well.
 This leads to the same issue with the foreign key constraints that we have seen
 in the plain partitioning approach we took.
 
+
 Foreign key considerations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -93,6 +94,22 @@ database is mostly about inserts and has no updates or deletes, we don't foresee
 much problems with this.
 
 
+Duplicated messages
+~~~~~~~~~~~~~~~~~~~
+
+When testing datagrepper and datanommer in our test instance with the timescaledb
+plugin, we saw a number of duplicated messages showing up in the `/raw` endpoint.
+Checking if we could fix this server side, we found out that the previous database
+schema had an `UNIQUE` constraint on `msg_id` field. However, with the timescaledb
+plugin, that constraint is now on both `msg_id` and `timestamp` fields, meaning
+a message can be inserted twice in the database if there is a little delay between
+the two inserts.
+
+However, migrating datanommer from fedmsg to fedora-messaging should resolve that
+issue client side as rabbitmq will ensure there is only one consumer at a time
+handling a message.
+
+
 Open questions
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