Repeat Alerts not working on the iphone 5
Just got my iPhone 5 2 days ago and noticed this as well.  Was on iOS 6.0.1 have since upgraded to 6.0.2 and same issue.

I've noticed if I leave a particular persons text messages up, I do not get another notification.  However, if I press the home screen and go back to the main screen I'll then receive audible notifications.

I did restore from my iPhone 4S backup via iTunes.  Later today I'm going to wipe the phone and rebuild from scratch and see if that helps.