I am not sure whether you know programming or not.
For a instance i can give you an idea for programming if you need all the help please ask.
You can create one table in the database with the following fields.
Id, username, Password, check
when ever any one login keep the check value is 1 and if the same username and password is entered again then check for the check field value if it is 1 then it show that the user is already login and another user is trying to login . then you can do what ever you want.
I hope this will help.
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Ananda Raj Pandey
http://www.softwebsnepal.com