Expanding on what @Canapin has said, in case you haven’t already figured it out, it should be straightforward to get the usernames unique since you’ve already established that racerx is the only duplicated username.
First, if you haven’t already, make sure racerx2 doesn’t already exist so you aren’t just moving the problem to a different username: (If you get an id, try a different username.)
SELECT id FROM users WHERE username_lower = 'racerx2';
Once you have an unused username, get the IDs of the duplicate racerx accounts:
SELECT id from users WHERE username_lower = 'racerx';
Pick one, doesn’t matter which though I would use the highest, then change its username: (Substituting racerx2 for a different username if needed and 12345 with the racerx ID you picked.)
UPDATE users SET username_lower = 'racerx2' WHERE id = 12345;