ASP.net c# Avoid open the same window twice

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C#

ASP.NET

Javascript

how to avoid to open same window of existing website(window) in new tab

for eg..
if i have a page www.mysite.com/page1.aspx if i copy this URL and paste in new tab of

the browser than a popup or message should be appear(alert-- you have already open this website)

Noe:-- if we reload or refresh the previous page than alter should not be appear .means only one page can be open at a time in should be done in scripting only

i need your valuable advice thanks in advance