I tried this fix and it worked for me. My S4 came with a messaging application that I have had no problems with, the icon is a white envelope with a yellow card coming out. When I updated recently my phone updated to the AT&T Messages and my phone automatically started using this app. That is when I started getting the duplicate messages as well as other little issues with my messaging. I went into my applications and disabled the AT&T messages and no more duplication!