Hi Kannan ,
The SAP system is working as expected . Is there any need that the same user should login from multiple terminals at the same time ?
You can limit the number of sessions opened under one user id using profile parameter ( rdisp/max_alt_modes , the default value should be 6 )
Multiple logons from the same user id does not necessarily lead to any performance issue .
Note : Same user id used for logon by multiple personnel is a license violation
Thanks ,
Manu