If you are getting repeat letters, slow down the typing speed. Go to accessibility settings/interaction and dexterity and experiment with slow keys, bounce keys, tap duration. See what works for you. You can also go to your keyboard settings. For Samsung keyboard, go to swipe touch and feedback and experiment with the various touch settings. For G-Board go to preferences and adjust the touch and hold duration. If it's just adding random letters, I don't have any suggestion for that, other than to switch to another keyboard and see if it still happens.
