iOS dictation duplicated
Surprisingly, I just figured it out!  I am using iPhone X 13.1.3 however, I think this would work on all iPhones and ios versions.
~ Go to settings
~Scroll down to Accessibility
~Scroll down to Voice Control and change it to OFF

I also had a microphone showing on my lock screen in the top left hand corner and this went away, as well as the duplicates when talking to text.  I tested it by switching it on and off to see if it fixed the problem and it did. Woohoo!  It was quite annoying.  I hope this helps :)