How to determine if a registered user already exists (Membership authentication)

MembershipCreateStatus Istatus; Membership.CreateUser (username, password, email, question, answer, true, out istatus);
Errirmsgtext.visible = true;
Switch (istatus)
{
Case Membershipcreatestatus.success:
if (request.querystring["ReturnUrl"]!= null)
{
FormsAuthentication.RedirectFromLoginPage (username, false);
}
Else
{
Formsauthentication.setauthcookie (Username,false);
Response.Redirect ("~/default.aspx");
}
Errirmsgtext.text = "New user created successfully. ";
Break
Case Membershipcreatestatus.invalidusername:
Errirmsgtext.text = "Invalid user name. ";
Break
Case Membershipcreatestatus.duplicateemail:
Errirmsgtext.text = "Duplicate e-mail address. ";
Break
Case Membershipcreatestatus.invalidquestion:
Errirmsgtext.text = "Invalid password hint problem. ";
Break
Case Membershipcreatestatus.duplicateusername:
Errirmsgtext.text = "The user name is registered, please replace the user name." ";
Break
Case Membershipcreatestatus.invalidanswer:
Errirmsgtext.text = "Invalid password answer. ";
Break
Case Membershipcreatestatus.invalidemail:
Errirmsgtext.text = "Invalid e-mail address. ";
Break
Case Membershipcreatestatus.invalidpassword:
Errirmsgtext.text = "Password must be 7 digits, and has 1 special symbols." ";
Break
Default
Errirmsgtext.text = "An unknown error occurred. Please contact the site administrator. ";
Break
}
MembershipCreateStatus Istatus;
This thing returns the results of your registered user. The following SWICTH code block gives the corresponding hints based on this result