Window.Open not working in Updatepanel

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C#2.0

C#

ASP.NET

I want to open the new window for each record which the check box is checked. This aspx page is inside the Update panel.Please check the following code, this code works for the first record which the check box is checked, but won't open the window for the next records which the check box is checked.

foreach (DataGridItem item in dgDuration.Items)
{
    CheckBox chkBox = (CheckBox)item.FindControl("cbstrChild");

    if (chkBox.Checked)
    {
        string script = "window.open('Report.aspx?Id=2&Cont=" + item.Cells[0].Text.Trim() + "', '');";
        Page page = HttpContext.Current.Handler as Page;
        if (page != null)
        {
            ScriptManager.RegisterStartupScript(this, this.GetType(), "NewWindow", script, true);
        }

    }
}

[edit]code block added[/edit]