Hi there
I’d be happy to help with this messaging issue!
Did this start suddenly, or has it always been this way? It sounds like an issue with the native app. I would recommend first ensuring you have read all texts and back them up if required. Then head to Application Manager from Settings. Swipe across to ALL and locate your native Messaging app. We first want to try Clear Cache. Power off the device, power on and send a text to 151 with the word TEST.
If the issue persists, return to the app in Application Manager and choose Clear Data. This will also erase any texts currently on the device. Again, restart the phone and test.
Should the duplicates remain, I would recommend performing a full backup and perform a factory reset by heading to Settings > Backup & Reset > Factory Data Reset.
I hope this helps!
Charise