I came up with a pretty awesome solution to this. What I've implemented was when user "Bob" logs in from their PC, and then the same user "Bob" logs in from another location, the log-in from the first location (their PC) will be killed while allowing the second log-in to live. Once a user logs in, it inserts a record into a custom table I created called "Logins". Upon a successful log-in, one record will be inserted into this table with values for "UserId, SessionId, and LoggedIn". UserId is pretty self-explanatory, SessionId is the current Session ID (explained below how to get), and LoggedIn is simply a Boolean that's initially set to True upon a successful user log-in. I place this "insert" logic inside my Login method of my AccountController upon successful validation of the user- see below:

Logins login = new Logins();
login.UserId = model.UserName;
login.SessionId = System.Web.HttpContext.Current.Session.SessionID;;
login.LoggedIn = true;

LoginsRepository repo = new LoginsRepository();
repo.InsertOrUpdate(login);
repo.Save();

For my situation, I want to place the check on each of my controllers to see if the currently logged in user is logged in elsewhere, and if so, kill the other session(s). Then, when the killed session tries to navigate anywhere I placed these checks on, it'll log them out and redirect them to the Log-in screen.

I have three main methods that does these checks:

IsYourLoginStillTrue(UserId, SessionId);
IsUserLoggedOnElsewhere(UserId, SessionId);
LogEveryoneElseOut(UserId, SessionId);

Save Session ID to Session["..."]

Before all of this though, I save the SessionID to the Session collection inside the AccountController, inside the Login ([HttpPost]) method:

if (Membership.ValidateUser(model.UserName, model.Password))
{
     Session["sessionid"] = System.Web.HttpContext.Current.Session.SessionID;
...

Controller Code

I then place logic inside my controllers to control the flow of the execution of these three methods. Notice below that if for some reason Session["sessionid"] is null, it'll just simply assign it a value of "empty". This is just in case for some reason it comes back as null:

public ActionResult Index()
{
    if (Session["sessionid"] == null)
        Session["sessionid"] = "empty";

    // check to see if your ID in the Logins table has LoggedIn = true - if so, continue, otherwise, redirect to Login page.
    if (OperationContext.IsYourLoginStillTrue(System.Web.HttpContext.Current.User.Identity.Name, Session["sessionid"].ToString()))
    {
        // check to see if your user ID is being used elsewhere under a different session ID
        if (!OperationContext.IsUserLoggedOnElsewhere(System.Web.HttpContext.Current.User.Identity.Name, Session["sessionid"].ToString()))
        {
            return View();
        }
        else
        {
            // if it is being used elsewhere, update all their Logins records to LoggedIn = false, except for your session ID
            OperationContext.LogEveryoneElseOut(System.Web.HttpContext.Current.User.Identity.Name, Session["sessionid"].ToString());
            return View();
        }
    }
    else
    {
        FormsAuthentication.SignOut();
        return RedirectToAction("Login", "Account");
    }
}

The Three Methods

These are the methods I use to check to see if YOU are still logged in (i.e. make sure you weren't kicked off by another log-in attempt), and if so, check to see if your User ID is logged in somewhere else, and if so, kick them off by simply setting their LoggedIn status to false in the Logins table.

public static bool IsYourLoginStillTrue(string userId, string sid)
{
    CapWorxQuikCapContext context = new CapWorxQuikCapContext();

    IEnumerable<Logins> logins = (from i in context.Logins
                                  where i.LoggedIn == true && i.UserId == userId && i.SessionId == sid
                                  select i).AsEnumerable();
    return logins.Any();
}

public static bool IsUserLoggedOnElsewhere(string userId, string sid)
{
    CapWorxQuikCapContext context = new CapWorxQuikCapContext();

    IEnumerable<Logins> logins = (from i in context.Logins
                                  where i.LoggedIn == true && i.UserId == userId && i.SessionId != sid
                                  select i).AsEnumerable();
    return logins.Any();
}

public static void LogEveryoneElseOut(string userId, string sid)
{
    CapWorxQuikCapContext context = new CapWorxQuikCapContext();

    IEnumerable<Logins> logins = (from i in context.Logins 
                                  where i.LoggedIn == true && i.UserId == userId && i.SessionId != sid // need to filter by user ID
                                  select i).AsEnumerable();

    foreach (Logins item in logins)
    {
        item.LoggedIn = false;
    }

    context.SaveChanges();
}

EDIT I just also want to add that this code ignores the capability of the "Remember Me" feature. My requirement didn't involve this feature (in fact, my customer didn't want to use it, for security reasons) so I just left it out. With some additional coding though, I'm pretty certain that this could be taken into consideration.