Merge pull request #40738 from vishal-gandhi/patch-1

Update iot-hub-devguide-messages-d2c.md

Author: American-Dipper

articles/iot-hub/iot-hub-devguide-messages-d2c.md

@@ -110,6 +110,12 @@ In addition to device telemetry, message routing also enables sending device twi
 
 When you create a new route or edit an existing route, you should test the route query with a sample message. You can test individual routes or test all routes at once and no messages are routed to the endpoints during the test. Azure portal, Azure Resource Manager, Azure PowerShell, and Azure CLI can be used for testing. Outcomes help identify whether the sample message matched the query, message did not match the query, or test couldn't run because the sample message or query syntax are incorrect. To learn more, see [Test Route](/rest/api/iothub/iothubresource/testroute) and [Test all routes](/rest/api/iothub/iothubresource/testallroutes).
 
+## Ordering guarantees with at least once delivery
+
+IoT Hub message routing guarantees ordered and at least once delivery of messages to the endpoints. This means that there can be duplicate messages and a series of messages can be retransmitted honoring the original message ordering. For example, if the original message order is [1,2,3,4], you could receive a message sequence like [1,2,1,2,3,1,2,3,4]. The ordering guarantee is that if you ever receive message [1], it would always be followed by [2,3,4].
+
+For handling message duplicates, we recommend stamping a unique identifier in the application properties of the message at the point of origin, which is usually a device or a module. The service consuming the messages can handle duplicate messages using this identifier.
+
 ## Latency
 
 When you route device-to-cloud telemetry messages using built-in endpoints, there is a slight increase in the end-to-end latency after the creation of the first route.