I noticed that a few other people have had this issue, but I would like to add mine as well. I have an iPhone 4S, iOS 6, on which I keep receiving duplicate text messages. I will receive one text (with the screen and sound notification), and then about 1 minute later, receive the same text (with screen and sound notification). Interestingly, when I open up Messages on my phone, I only see one of them, not two identical texts. The texts are usually green and they are usually sent by SMS (from people who don't have smartphones). Not every text I receive comes in duplicate but over half of them do.
This is more of an annoyance than anything, but I'd also like to know that I'm not getting charged for two texts out of my allowance rather than one.
If you can provide any help, I'd appreciate it.