How do I turn off repeat message alerts on iPhone?
Default settings on the iPhone with iOS 5 or iOS 6 installed are set up so that when an incoming text or MMS message arrives, the device reminds you with two alerts. This repeated alert every 2 minutes can make sure you know a message came in, or it can be really annoying as you think the iPhone is receiving multiple SMS alerts in a row.
To switch off these reminder alerts, follow these steps:
1. Navigate to Settings -> Notifications -> Messages
2. Scroll to the bottom of the page and tap Repeat Alert
3. Select how many times you want to be reminded of each text message
Setting this to Never will ensure that your iPhone only plays the alert tone once for each message received.