Duplicate user entries in aspnet_Users table

 I'm using SqlMembershipProvider and SqlRoleProvider for user management.
Users are created by
Dim MCS As System.Web.Security.MembershipCreateStatus
Dim NewUser As MembershipUser = Membership.CreateUser(TB_UserName.Text, TB_Pw.Text, TB_Email.Text, TB_SecQuestion.Text, TB_SecAnswer.Text, False, MCS)
later, after email confirmation, I take the user into a specified role

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