iPhone message alert again as I open phone

A really annoying thing that has been happening over last few months for me is that I receive a text msg alert and a few moments later (sometimes it is longer) I open my phone to read the msg and it sounds the msg alert again. This is sooooo annoying as I already know I have a msg - but just made the msg alert noise, so why is it doing it again when I open the phone to read the msg a few moments later. Ay thoughts on how to get rid of this?