Only one concurrent login per UserID in Asp.net

Please have look
One login per user[^]

limit-only-one-session-per-user-in-asp-net[^]

Just do one thing. In your database, in your usertable, add one bit field called IsLoggedin. when user login check for true or false value. If value is true(i.e. already logged in), then automatically redirect to home page. and if value is false(not logged in), then update that value to true, and redirect.
Refer this
[^]

You need to use session management in this. Store the username as sessionID variable.
the Abandon method should work like Session.Abandon();
If you want to remove a specific item from the session use something like Session.Remove("YourItem");
And if you are using this Database approach to track User login status, after ending session, set the flag to 0(logged out) again.

Hope it helps :)