add notes on external event duplication

Author: davidmrdavid

articles/azure-functions/durable/durable-functions-best-practice-reference.md

@@ -71,6 +71,10 @@ A single worker instance can execute multiple work items concurrently to increas
 > [!NOTE]
 > This is not a replacement for fine-tuning the performance and concurrency settings of your language runtime in Azure Functions. The Durable Functions concurrency settings only determine how much work can be assigned to a given VM at a time, but it does not determine the degree of parallelism in processing that work inside the VM. The latter requires fine-tuning the language runtime performance settings.
 
+### Create IDs for your external events so they may be de-duplicated
+
+As with Activity Functions, external events have an _at-least-once_ delivery guarantee. This means that, under certain conditions (like restarts, scaling, crashes, etc.), your application may receive duplicates of the same external event. Therefore, we recommend that external events contain some kind of ID that allows them to be manually de-duplicated in orchestrators.
+
 ### Invest in stress testing
 
 As with anything performance related, the ideal concurrency settings and architechture of your app ultimately depends on your application's workload. Therefore, it's recommended that users to invest in a performance testing harness that simulates their expected workload and to use it to run performance and reliability experiments for their app.

articles/azure-functions/durable/durable-functions-external-events.md

@@ -329,6 +329,8 @@ The *"wait-for-external-event"* API waits indefinitely for some input.  The func
 > [!NOTE]
 > If your function app uses the Consumption Plan, no billing charges are incurred while an orchestrator function is awaiting an external event task, no matter how long it waits.
 
+As with Activity Functions, external events have an _at-least-once_ delivery guarantee. This means that, under certain conditions (like restarts, scaling, crashes, etc.), your application may receive duplicates of the same external event. Therefore, we recommend that external events contain some kind of ID that allows them to be manually de-duplicated in orchestrators.
+
 ## Send events
 
 You can use the *"raise-event"* API defined by the [orchestration client](durable-functions-bindings.md#orchestration-client) binding to send an external event to an orchestration. You can also use the built-in [raise event HTTP API](durable-functions-http-api.md#raise-event) to send an external event to an orchestration.