title: Asp.Net Identity 2 duplicate username check
body: <p>UserManager in Asp.Net Identity 2 prevents creation user with duplicate username through additional request to database to find possible duplicate. I think this is error prone and can cause concurrency errors. The correct mechanism should rely on on unique constraints or indexes. Am I wrong and do I miss something?</p>
  
  <p>Links to source:
  <a href="https://github.com/aspnet/Identity/blob/2c9f43a16045d395b8dcc68ea72d9c8d7860e008/src/Microsoft.AspNet.Identity/UserManager.cs#L300" rel="nofollow">CreateAsync</a> and <a href="https://github.com/aspnet/Identity/blob/57002ba35960fcc58380b52229695e7f6d1975c8/src/Microsoft.AspNet.Identity/UserValidator.cs#L62" rel="nofollow">ValidateUserName</a></p>