I too am having this issue...but on ATT. However, not with email, only with texting, both SMS and MMS. I'm using the stock Text app, no root...bone stock. I receive duplicates, sometimes 15 mins later, and send multiple texts as well.
The sending icon shows for quite a while, which is when i'm sure the error occurs. I went to my local ATT store and they said it was a SIM card issue...whoever performed my upgrade gave me a SIM card for the iPhone rather than Android/Windows 4g LTE. I had no idea that there are different SIMS for different mobile platforms, so i checked the ATT site and sure enough, there are.
I don't get the duplicates as often as i did before...it was at least 30%-50% of my texts before the SIM card change, where as now, it is <5%. Still, that is unsatisfactory.
Since you were having this issue on the Sprint network, i'm starting to think this is more of a handset software/hardware issue...i'm hoping it's software.