Receiving push notifications twice

We have been using Firebase to send push notifications for a couple of months in our app, and I have used it in other of my apps . Everything had gone well, however a week ago we started receiving duplicate notifications (not always). The strange thing is that we did not change any code, it just started to happen in both Android and iOS. It only happens sometimes not always, and only in some users/devices.

I want to know if someone else is going through this with Firebase. I think it is a Firebase bug but I would like to see if someone can confirm something about it.

The ios app is made in swift, and the android app in Java.

BTW. We are sending it based on this with FCM in our back-end. And we have no duplicated topics. I have checked and devices are only subscribed once.

https://fcm.googleapis.com/fcm/send

Content-Type:application/json
Authorization:key=AIzaSyZ-1u...0GBYzPu7Udno5aA
{
  "to": "/topics/foo-bar",
  "data": {
    "message": "This is a Firebase Cloud Messaging Topic Message!",
   }
}

Here my c# code in our back-end to send notifications

public void sendPushNotification(string _title, string _message, string _topic, object customData = null)
    {
        var requestUri = "https://fcm.googleapis.com/fcm/send";

        WebRequest webRequest = WebRequest.Create(requestUri);
        webRequest.Method = "POST";
        webRequest.Headers.Add(string.Format("Authorization: key={0}", FCM_SERVER_API_KEY));
        webRequest.Headers.Add(string.Format("Sender: id={0}", FCM_SENDER_ID));
        webRequest.ContentType = "application/json";

        var data = new
        {
            to = "/topics/" + _topic, // this is for topic 
            notification = new
            {
                title = _title,
                body = _message,
                sound = "default"
            },
            data = customData, //custom data (got it from firebase documentation)
            content_available = true 
        };

        var serializer = new JavaScriptSerializer();
        var json = serializer.Serialize(data);
        Byte[] byteArray = Encoding.UTF8.GetBytes(json);
        webRequest.ContentLength = byteArray.Length;
        using (Stream dataStream = webRequest.GetRequestStream())
        {
            dataStream.Write(byteArray, 0, byteArray.Length);

            using (WebResponse webResponse = webRequest.GetResponse())
            {
                using (Stream dataStreamResponse = webResponse.GetResponseStream())
                {
                    using (StreamReader tReader = new StreamReader(dataStreamResponse))
                    {
                        String sResponseFromServer = tReader.ReadToEnd();
                    }
                }
            }
        }
    }