Update: I solved it by setting
            .setGroupAlertBehavior(GROUP_ALERT_SUMMARY)
I am creating notifications like
new NotificationCompat.Builder(context, channel_id)
            .setColor(..)
            .setContentTitle(..)
            .setSubText(..)
            .setSmallIcon(..)
            .setAutoCancel(true)
            .setGroup(groupKey)
            .setGroupSummary(true)
            .setContentIntent(resultPendingIntent)
            .build();
On 8.0 some devices are seeing double notification sound for every notification. There is a blog post around this
Some apps seem to have figured out a way to solve the issue? Help!