How to create popup login window using 'ClientScript.IsStartupScriptRegistered'

I'm using this ClientScript to make frmLogin as popup window. After I click login, the widow will close and direct to frmForm2 in master web form.

frmForm1

If (Not ClientScript.IsStartupScriptRegistered(Me.GetType(), someScript)) Then
                ClientScript.RegisterStartupScript(Me.GetType(), someScript, "window.open('Klien/frmLogin.aspx?stsemak=1','" & Session("strNoMatrik") & "','" & Session("strIdKaunseling") & "');", True)
            End If

frmLogin

If (Not ClientScript.IsStartupScriptRegistered(Me.GetType(), someScript)) Then
                            ClientScript.RegisterStartupScript(Me.GetType(), someScript, "window.location.replace('frmForm2.aspx','" & Session("strIdKaunseling") & "','" & Session("strNoMatrik") & "'),'" & Me.hidIDKaunseling.Value & "';", True)
                        End If

The problem here, when I clicked login, frmForm2 display at the same window. So meaning it will display 2 windows. I hope someone can help me ASAP. TQ