Form Login VB.net เราจะทำยังไงให้ User Login ซ้อนกันไม่ได้

Public Class frmLogin

    Private Sub btnLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLogin.Click

        Dim objConn As New MySqlConnection
        Dim objCmd As New MySqlCommand
        Dim strConnString, strSQL As String

        strConnString = "Server=db4free.net;User Id=xxxx; Password=xxxx; Database=xxxx; Pooling=false"
        objConn.ConnectionString = strConnString
        objConn.Open()

        Dim intNumRows As Integer
        strSQL = "SELECT COUNT(*) FROM member WHERE Username = '" & Me.txtUsername.Text & "' AND Password = '" & Me.txtPassword.Text & "' "
        objCmd = New MySqlCommand(strSQL, objConn)
        intNumRows = objCmd.ExecuteScalar()

        If intNumRows > 0 Then
            Dim frm As New frmMain
            frm._strUser = Me.txtUsername.Text
            frm.LoadInfor()
            MsgBox("แอแอแอ!!!")
            MsgBox("อิอิ")
            frm.Show()

            Me.Hide()
        Else
            MessageBox.Show("แอร๊ย")
        End If

        objConn.Close()
        objConn = Nothing

    End Sub