Maybe there is an issue with the migration tool that should be reported. In order to verify that the user doesn't exist in the database, could you please access MySQL and obtain the users that exist in the database? Please note that I'm going to share the commands that need to be run inside a Bitnami installation. If you are not using a Bitnami installation right now, you will need to adapt those commands to the configuration you have.
# Command to access MySQL
mysql -u root -p
# Command to obtain the username and email of all the users in the database
use bitnami_wordpress; select user_login,user_email from wp_users;