Membership create inserting duplicate username in aspnet_Users table

I have the following code, which is creating duplicate username with different UserId in the aspnet_Users table -

Dim User As MembershipUser

Dim Status As MembershipCreateStatus

Try
User = Membership.CreateUser(txtUserId.Text.ToString(), txtPassword.ToString(), txtEmail.Text.ToString(), Nothing, Nothing, True, Status)
If Status = MembershipCreateStatus.Success Go to the complete details ...