Check if my website is open in another tab

The shorter version with localStorage and Storage listener

<script type="text/javascript">
        // Broadcast that you're opening a page.
        localStorage.openpages = Date.now();
        var onLocalStorageEvent = function(e){
            if(e.key == "openpages"){
                // Listen if anybody else is opening the same page!
                localStorage.page_available = Date.now();
            }
            if(e.key == "page_available"){
                alert("One more page already open");
            }
        };
        window.addEventListener('storage', onLocalStorageEvent, false);
</script>

Update:

• Works on page crash as well.
• Stimulate page crash in chrome: chrome://inducebrowsercrashforrealz

Live demo