mysql update user details if they used the same string for username and password

Hi i have an issue in my database i opened a test site up for users to register i now have 200,000 registered users sadly i forgot to enable some check and a lot of users have registered with the same details for username and password

is there any way in mysql to update the passwords for users to a random string if they have the same string for there username and there password ?

example of this i hope for something like

update users set password="RANDOM STRING" WHERE username=passsword