The JavaScript exception persists because the WebTab object finds that TabPage with specified name “xyz” already exist. In order to block this error, you can try to add a validation that will check whether TabPage with specified name “xyz” already exist or not before invoking the AddNewTabItem() method.

The script below is shows the modified version of your AddNewTab JavaScript function.

function AddNewTab(name) {
    var tab = ISGetObject("WebTab2");

    if (!tab.TabPages.GetNamedItem(name)) {
        var newItem = tab.AddNewTabItem(name, name, null);
        var activeTab = newItem;

        activeTab.ContentMode = "UseIFrame";
        newItem.SetActive(); // set the newly created tab item as active

        // insert reference in newly created tab
        setTimeout(function () { activeTab.SetContentURL(name + ".aspx"); }, 50);
    }
    else {
        // 1.Raise a simple alert that simply tells the user:
        //   "This tab is already open"; and/or
        alert("This tab is already open");

        // 2.Simply return the focus on the existing tab, if found in the colletion,
        //   instead of trying to create it.
        tab.TabPages.GetNamedItem(name).SetActive();
    }
}

Hope this helps.