If you attempt to sign up with an already existing user, it doesn't let you, but it doesn't show the error message of "Account with that email address or username already exists" on being redirected back to the sign-up page. If you refresh the page then, the error does appear.
If you attempt to sign up with an already existing user, it doesn't let you, but it doesn't show the error message of "Account with that email address or username already exists" on being redirected back to the sign-up page. If you refresh the page then, the error does appear.