title: Duplicate FCM push notifications
body: <p>Today, I received duplicate FCM push notifications over a period of ~30 mins. These push notifications are identical in terms of content, and the push notification was sent ONCE. Here's a snippet of the timeline of when I was receiving these push notifications:</p>
  
  <pre><code>1   01-15 10:41:30.349+0200 MyPushListenerService   onMessageReceived
  1   01-15 10:43:30.004+0200 MyPushListenerService   onMessageReceived
  1   01-15 10:47:31.665+0200 MyPushListenerService   onMessageReceived
  1   01-15 10:55:32.062+0200 MyPushListenerService   onMessageReceived
  </code></pre>
  
  <p>Why does this happen?</p>
  
  <p>How does Firebase confirm that a push notification was delivered? Are there retry mechanism?</p>