My wife and I both run into a problem with some regularity on our iOS phones where the dictation (via the microphone button off stock iOS keyboard) will get duplicated. Often we dictate 10-25 words and suddenly it has inserted them all twice, as if we repeated what we had said a second time. Really annoying and have seen this for at least a year. We have seen it with both emails and texts. Appreciate any suggestions or a link to a public Radar ticket we can up-vote.
iOS version 12.4.1 currently, but has happened on earlier versions.
(There was a laughable suggestion on Apple support community that sounded like a Windows support call from 2002: "reboot phone, then reinstall iOS, and see if that fixes it.")