Hi there PhinnM,
If your text is getting duplicated when using Dictation on your iPhone, first follow these steps from If the microphones on your iPhone aren't working to clear the microphones of your device since this can cause issues like this to occur:
Clear the microphones
1. Remove any screen protectors, films, or cases from your iPhone.
2. Clear any debris from the openings of your iPhone.
Let's also ensure your iPhone is fully updated since updates install feature improvements and fixes that can resolve unexpected behavior. This article walks you through how to back up and update: Update your iPhone or iPad
If this continues after clearing the microphones and updating your iPhone, then reach out to Apple Support next. They would be the best equipped to investigate this further. Contact them here: Get Support
Cheers!