I get this too with non iMessage texts. It's seems like it's been happening more lately. But what I did notice is that it mostly happens when I am at work. The coverage there like the Bermuda Triangle of cell service It's awful at it's best no matter if I'm in the building or outside. We have wifi, but that doesn't help with regular text messages. I have a M-Cell at home and rarely get duplicate texts when I'm home. See if you have the issue in a particular location and maybe it's just the coverage where you are.