I have been experiencing the problem with Messages (default txt app on phone), Hancent and I even tried Hangouts. Still same thing.
I get the delay when sending to anyone, not just the person that says he received duplicates. Its possible other ppl are get multiple txts from me as well but are not telling me. He is aware of my issue and I told him to tell me when he receives duplicates, and its always after a delayed msg.
My buddy that I am texting has an LG G2. No issues from anyone else that he receives msgs from. It its always on the delayed text msg that he gets the duplicates.
Ik its not my phone intentionally sending two because I have all other msg apps disabled when using hancent (primary app I use). Also I have heard it from a few others that they received duplicates from me, always after I have trouble sending them.
The only option I have found to fix this is for me to restart my phone and that will usually result in me being able to send some msgs without delay.
I havent had any messages rejected (sending) yet. Which has happened before. They are just very delayed in sending, sometimes as much as 3-5 minutes.
VZW apparently has no fix and no suggestions. It only started happening when I got my new phone (m8) and a new sim.