After all of the iterations of iOS, there is still no way to have only the first new/unread message per SMS/iMessage thread generate a notification--particularly the notification sound or tone.

I find the feature would be useful in all situations, but would be especially useful in group threads. I don't need to hear a ding every time someone in a group thread sends a message if I don't have my phone in my hand. One ding generated when the first unread message is received will do just fine.