Stop people having my website loaded on multiple tabs

I’ve created a simple solution for this. The master page layout creates a tab GUID and stores it in sessionStorage area of the tab. The using an event listener on the storage area I write the tab GUID to the sites localStorage area. The listener then compares the tabs GUID to the one written to site storage and if they differ then it knows more than one tab is open.

So if I have three tabs A,B,C then click something in tab C, tab A and B detect another tab is open and warn user of this. I haven’t yet got to fixing it so the last tab used get’s notification, work in progress.

Here’s the JS I have in master page, plus in the login page I have a localStorage.Clear to clear last tab from previous session.

    // multi tab detection
function register_tab_GUID() {
    // detect local storage available
    if (typeof (Storage) !== "undefined") {
        // get (set if not) tab GUID and store in tab session
        if (sessionStorage["tabGUID"] == null) sessionStorage["tabGUID"] = tab_GUID();
        var guid = sessionStorage["tabGUID"];

        // add eventlistener to local storage
        window.addEventListener("storage", storage_Handler, false);

        // set tab GUID in local storage
        localStorage["tabGUID"] = guid;
    }
}

function storage_Handler(e) {
    // if tabGUID does not match then more than one tab and GUID
    if (e.key == 'tabGUID') {
        if (e.oldValue != e.newValue) tab_Warning();
    }
}

function tab_GUID() {
    function s4() {
        return Math.floor((1 + Math.random()) * 0x10000)
          .toString(16)
          .substring(1);
    }
    return s4() + s4() + '-' + s4() + '-' + s4() + '-' +
      s4() + '-' + s4() + s4() + s4();
}

function tab_Warning() {
    alert("Another tab is open!");
}

Note: It’s IE9+

Hope this helps.