How to disable notification sound that keeps repeating after I get a message

Can anyone tell me where and what settings to go to, so when I get a new message I just hear one message tone, once. Somehow I've changed something and now when I get a new message I hear a message tone, just once, but then it's followed by a different tone which keeps repeating, every 30 seconds or so, until I check the notification screen and acknowledge my message there as well. It's so frustrating if I can't get to the phone, like when I'm driving, please help me!?