Check For Existing Window

I need to check for a particular window which I opened as a popup,
already exists, before I try to open the same again. I achieved that
with the following code. So this is fine.

var myWin = null;
function openPopup(aURL){
if (( myWin==null) || myWin.closed) {
myWin = open(aURL,"myWin");
}

/* Window is already opn, thus maps the url to the same window
and give focus */
else {
myWin.location = aURL;
myWin.focus();
}
}

Now my problem is I need to do a similar check for a page thats been
href to the top.location
top.location=aURL;

In this situation if this same code runs again, i need to check if this
window is already open, and if so not to open it. Can it be done in a
similar way like the above.