It was kinda off putting with my new iPhone 4S to find out that Apple makes SMS alerts ring twice (once on receive and one reminder) instead of having an LED light for alerts like in Android or Blackberry. Kind of a workaround rather than a real solution to the problem of how to find out I have alerts pending when I missed the first alert. Apple operates on a push principle and Google/RIM operates on pull.
Anyway, if I have reason to miss the first one, I most likely won’t notice the second one. It’s also misleading when it rings twice because you’d think there’s 2 messages.
To turn that functionality off, go to Settings->Notifications->Messages->Repeat Alert->Never.