I have this problem as well with iPhone 4, iOS 5.0.1.
It seems that when the conversation history on a particular contact gets too long, instead of sending out the new message it sends out the previous message a second time. Messages shows that the new text message was sent, so you only find out after getting a confused message back from the recipient.
After clearing all messages from that particular contact, everything worked ok for a while before again starting to send previous message again. Clearing all helped again, though this doesn't seem to be ideal.
I am having this (or similar) problem. I send a text that is less than 160 characters and my 4s either breaks the message into two or sends a second message labled "duplicate" on the other end.
I'll try clearing the history bnt this seems like it might be something that needs to be fixed by Apple (hoping...).
How would reseting the Network Settings help? I'm reluctant to reset anything to default.
I have tried powercycling the phone and I tried deleting the text message history and neither made a difference.
I have not yet tried going to Sprint about the problem because I don't have that kind of time and it makes me sad how little they actually know there.
This is really getting frustrating... :/
When I had the issue with iOS5.0.1 it didn't look like a carrier issue. SMS to all other contacts worked fine and reset network settings didn't seem to help.
Since updating to iOS 5.1 I've not seen the issue again, so I'd guess my mucky work around may not help any more.
Re reading hWomBat's issue - I think it is less likely that this is the same issue. I agree with diesel vdub that an issue with splitting SMS or immediate redelivery could be carrier related.
The issue I was experiencing was that the previous message would be sent instead of message just written. The previous message could have been sent and received days before.
That info is just as useless as the info from 02 lol, reboot device, update firmware, reset network settings, bla bla bla, I've heard them all as solutions to every problem I've had and it's never worked for any of them,
I've had no 3G for weeks O2 said it was because I was on ios6 (I lied) as they originally said it was because I didn't update to ios6, then they had me resetting settings even tho I told them problem was on their end, sure enough 100+ minutes of phonecalls and they finally admitted 4 of their transmitters in my area were damaged due to floods! Would of been easier to tell me that in the first place, they won't even speak to me unless I said I had updated to the latest software as they blame everything on apple, and if you do update they then blame it on updating!
My issue is my wife sends me a SMS and I get it twice within seconds of each other
I'm running 5.1.1 shes running 6.0
Problem was here when we both had 5.0
Problem was here when we both had 5.1
So it's not a firmware problem and apple has done diddly squat to fix it
The first is a text message the second is an iMessage both together or within seconds of each other
Pretty sure its her 3G connection at work failing to send the iMessage then sending a text message instead but then sending the iMessage at the same time because it just so happened to connect
No need to reset anything