Vbnet SQL中的重复条目

我一直输入相同的内容username，但仍保存到DB中。我的代码有什么问题？

sql = "INSERT INTO testing_mysql_vb(id,user_name) VALUES(NULL,'" & TextBox1.Text & "')"

    Try
        dbcomm = New MySqlCommand(sql, dbconn)
        dbread = dbcomm.ExecuteReader()
        dbread.Close()

        If dbread.HasRows Then
            sql = "select from testing_mysql_vb where user_name = '" & TextBox1.Text & "'"
            MsgBox("Duplicate record!")

        End If

    Catch ex As Exception
        MsgBox("Error in saving to Database. Error is :" & ex.Message)
        dbread.Close()
        Exit Sub
    End Try
    MsgBox("The User Name was saved.")
    TextBox1.Text = ""