Check if a page is opened . IF opened, close the page.

Hi!
I am generating a new report page by using the following code.
ScriptManager.RegisterStartupScript(this, typeof(string), "openNewWindow", "window.open('../Report_Preview.aspx','ReportPreview','scrollbars=yes,resizable=yes,')", true);
 
 But, the above script is used in more than 20 parameter page which ultimately always open the same Report_preview.Aspx page.
What i want to check  while running the above script is "give a warning if the Report_preview.Aspx is already opened"
 
Any idea please!!!!!
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