By default, all WordPress content-related tables are dropped and recreated as part of the migration process, wiping out all the existing data, and thus, avoiding conflicts with any sample data entered by before the migration.
The same process can not be employed when migrating users. Otherwise, the migration would remove the admin user information entered when installing WordPress and you would not be able to log in your own site. This is why, the .sql script containing the user migration data adds the new users, but respects the existing admin user in the WordPress site.
Nevertheless, the script assumes that the admin user (id = 1) is the only existing WordPress user. If there are other users please remove them before importing the file to avoid an error importing a WordPress users file. Alternatively, you could provide us with a copy of your WordPress database and we could configure the migration to make sure it respects all existing content.
Posted in: Content export / import Error importing a WordPress users file