By default, when you get an SMS or iMessage, your iPhone will make a sound once when you receive it, and then again two minutes later in case you missed it. If you read the message after the first ding, it doesn’t ding again.
This is a nice feature if you’re the kind of person who regularly misses notifications. If, on the other hand, you’re always pretty aware of your phone, then the odds are the second sound is just going to annoy you; you didn’t read the message yet because you chose not to.
Here’s how to turn the feature off, or if you’re really crazy, get the notification to repeat even more.
Go to Settings > Notifications > Messages and scroll down.
Select Repeat Alerts and then change it from Once to Never, or however many times you want the alert to repeat.
Remember, this is the number of times you want the notification to repeat, so Never means you get a single notification when the message arrives and 10 Times means you get 11 notifications: once when the message arrives, and then 10 more every two minutes.