How to prevent from log in same username from different system at a time?

Question

hello, i have developed several mlm applications for my local client. now i need to prevent from log in same username from different system at a time. i need this badly to solve my client's problem. i have developed an idea when login update a log sheet as logged and log out update same entry as logged out but this logic fails when power failure or browser closed. so can anyone suggest me to better solution or sending a query to my sql server after power failure or application close? please help me..... i am in big trouble...thank you for reading this.