ORA-02391: exceeded simultaneous SESSIONS_PER_USER limit

ORA-02391: exceeded simultaneous SESSIONS_PER_USER limit
  *Cause: An attempt was made to exceed the maximum number of
          concurrent sessions allowed by the SESSION_PER_USER clause
          of the user prfile.
  *Action: End one or more concurrent sessions or ask the database
           administrator to increase the SESSION_PER_USER limit of
           the user profile.