One of the wacky things you may notice if you’ve just gotten a new iPhone is the default double alert whenever you get a text message, whether iMessage or SMS. Why Apple has this as the default, I’m not sure, but it kept freaking me out before I figured out how to turn it off.
However, I’m willing to see that you might want the double alert, or more (shudder), and there’s a simple way to make that happen, as well.
Launch your Settings app, and tap on Notifications. Tap on Messages, then scroll down to where it says, Repeat Alert. Tap there to open the preference pane.
Here, you can tap on Never, Once, Twice, 3 Times, 5 Times, or 10 Times. Note that if you tap on Once, you’ll get the original alert for the text message, and then one Repeat alert. If you tap on Twice, you’ll get the original alert, and then two repeat alerts. And so on.
Personally, I leave this set to Never, since I always think the repeated alert is another text message coming in and it adds to my stress level. However, if you need to be reminded more than once, have at it!
Via: OS X Daily.