title: Why can't I collapse my push notifications when I use Firebase FCM?
body: <pre><code>const options = {
      priority: 'high',
      collapseKey: user_id
  };
  const deviceTokensPromise = db.ref('/users-fcm-tokens/' + user_id).once('value');
  deviceTokensPromise.then(tokensSnapshot =&gt; {
      if (!tokensSnapshot.hasChildren()) {
          return console.log('There are no device tokens to send to.');
      }
      const tokens = Object.keys(tokensSnapshot.val());
      console.log(tokens);
      console.log(payload);
       return admin.messaging().sendToDevice(tokens, payload, options).then(response =&gt; {
           console.log(response);
           return removeInvalidFCMTokens(tokensSnapshot, response);
       });
  });
  </code></pre>
  
  <p>I have a collapse-Key field in my options.</p>
  
  <p>When this code is ran, the iPhone receives multiple notifications, all on top of each other. I'd like to have most recent notification replace the previous ones.</p>