Check if a popup window is already open before opening it using Jquery

I want to check if a popup window is already open , before I open the popup window. How do I get it done using Jquery?

Below is my code to open a new popup window :

window.open("mystopchat.php?stat=1&session="+data['myid1']['session_id'][i],"win1","width=500,height=500"); 

Now before I call this, I want to be sure that this popup window is not already open.