Maybe this is old but I've realized that if I receive a text and don't check it, some minutes later a text alert sound will go off (the screen won't be on, though), wait some more and the alert will go off again. I don't know how many minutes it is, but I know it happens twice.
I don't know if it happens more than that because I usually check it after the second.
I don't know if it happens more than that because I usually check it after the second.