Firebase Cloud Messaging - Upstream messages are received twice - firebase

Question

Our app sends some messages to our servers via FCM Upstream. We have two servers connected as clients.
Recently, we've noticed that many messages (>50%) are received twice. Often on different servers, but sometimes on the same server.
Unsure where to go from here.
Logs of one message being received twice follow:
Fri, 18 Nov 2016 21:39:17 GMT xmpp:connection receive:
<message to="xxxx#gcm.googleapis.com" from="devices#gcm.googleapis.com" type="normal">
<gcm xmlns="google:mobile:data">
{"data":
{"userInfo":"{}",
"path":"v1\/patient\/checkIn\/usage",
"method":"POST",
"body":"{}",
"dest":"https:\/\/api.stg.triggrhealth.com\/api\/v1"
},
"time_to_live":43200,
"from":"f5LDS516Eu8:APA91bGEkzwXlmXRpfui6T01QM6V3pOY8p3nSkZMUfYkDYjRxu6Xa64Bh0Boc3so8X8royBA1v4PaGjPcUW6JI0pcZL7UK9P4LEqgVThfNQKGW8DZMhTEM-JDodDvUPjGpbPA-zdYYQb",
"message_id":"1479505157658",
"category":"com.triggrhealth.prototype.stage"
}
</gcm>
</message>
Fri, 18 Nov 2016 21:39:17 GMT xmpp:connection send:
<gcm xmlns="google:mobile:data">
{"to":"f5LDS516Eu8:APA91bGEkzwXlmXRpfui6T01QM6V3pOY8p3nSkZMUfYkDYjRxu6Xa64Bh0Boc3so8X8royBA1v4PaGjPcUW6JI0pcZL7UK9P4LEqgVThfNQKGW8DZMhTEM-JDodDvUPjGpbPA-zdYYQb",
"message_id":"1479505157658",
"message_type":"ack"
}
</gcm>
Fri, 18 Nov 2016 21:42:28 GMT xmpp:connection receive:
<message to="xxxx#gcm.googleapis.com" from="devices#gcm.googleapis.com" type="normal">
<gcm xmlns="google:mobile:data">
{"data":
{"userInfo":"{}",
"path":"v1\/patient\/checkIn\/usage",
"method":"POST",
"body":"{}",
"dest":"https:\/\/api.stg.triggrhealth.com\/api\/v1"
},
"time_to_live":43200,
"from":"f5LDS516Eu8:APA91bGEkzwXlmXRpfui6T01QM6V3pOY8p3nSkZMUfYkDYjRxu6Xa64Bh0Boc3so8X8royBA1v4PaGjPcUW6JI0pcZL7UK9P4LEqgVThfNQKGW8DZMhTEM-JDodDvUPjGpbPA-zdYYQb",
"message_id":"1479505157658",
"category":"com.triggrhealth.prototype.stage"
}
</gcm>
</message>
Fri, 18 Nov 2016 21:42:28 GMT xmpp:connection send:
<gcm xmlns="google:mobile:data">
{"to":"f5LDS516Eu8:APA91bGEkzwXlmXRpfui6T01QM6V3pOY8p3nSkZMUfYkDYjRxu6Xa64Bh0Boc3so8X8royBA1v4PaGjPcUW6JI0pcZL7UK9P4LEqgVThfNQKGW8DZMhTEM-JDodDvUPjGpbPA-zdYYQb",
"message_id":"1479505157658",
"message_type":"ack"
}
</gcm>

Answer 1

You may want to check and try sending an ACK message. As mentioned,
In response to an upstream message, the app server must use the same connection to send an ACK message containing the unique message ID. If GCM does not receive an ACK, it may retry sending the message to the app server.
Here's the sample code:
<message id="">
<gcm xmlns="google:mobile:data">
{
"to":"REGID",
"message_id":"m-123"
"message_type":"ack"
}
</gcm>
</message>
Lastly, please also see the XMPP Connection Server Reference for more information about upstream message syntax.