If you are using Basepage logic its better to use this code there.

Redirection logic is written for pages with Frames or without Frames

[sourcecode language='vb']

Protected Overrides Sub OnPreRender(ByVal e As System.EventArgs) MyBase.OnPreRender(e)
If context.Session.IsNewSession = True Then
Dim strCookieHeader As String = Page.Request.Headers(“Cookie”)
If Not strCookieHeader Is Nothing Then
strCookieHeader = strCookieHeader.ToLower
End If

‘Checking for duplicate Session.
If Not strCookieHeader Is Nothing AndAlso strCookieHeader.IndexOf(“asp.net_sessionid”) >= 0 Then
If Page.Request.IsAuthenticated = True Then
System.Web.Security.FormsAuthentication.SignOut()
End If

‘If you want to redirect to defaultpage you can use this
‘Page.Response.Redirect(“default.aspx?Timeout”)

‘If you are using Frames then this will work.
Response.Write(“window.open(‘default.aspx?Timeout’,'_parent’);”)
End If
End If
End Sub

[/sourcecode]