title: I can't install WordPress on an existing database
body: <p>Today I was trying to install a new copy of WordPress on my local machine. There was a working copy of a site in there that I accidentally deleted. So I tried to reinstall WordPress on that existing database. Everything was fine, but in the last stage of the installation a weird message was coming up. It was saying <code>Please provide a valid username.</code> and there wasn't a field to provide a username, instead the text <code>User(s) already exists.</code> I have never come around this and neither have I heard of it. I am curious to know why WordPress is wanting a username when the field is locked. Is this a bug or something else?</p>
  
  <p>Thanks in advance.</p>