Occasional voice to iPhone glitch

At irregular intervals, when I speak to iPhone, the dictated text, whether accurate or not, is repeated 2 or 3 or even more times.  This happens at odd intervals, not very often. 

 I can think of no reason why this should happen. My iPhone is model 6s Plus. It has not been left out in the sun, dropped from a great height, immersed in any liquid, jailbroken, or otherwise abused. It was not pre-owned either. It works on the most recent IOS, and I have had it for about 3 years. It gets a lot of use, so I am wondering whether this glitch is a sign of age. I recharge it at least once a day. Overall, the device functions smoothly enough. 

Any advice that could help to resolve this problem would be gratefully received.