how to avoid multiple session for the same username

[tubin gen]

I need suggestions on implementing single user login , like My system has a
user with a username    jdavid and my application should not allow two
different sessions for the same username jdavid , can  one session  peep
into all other session
to see if the any has a  usernaem jdavid, I am basically looking for some
pattern if  any available for  restricting  multiple session for the same
username .