This is a critical and security feature. You can know whether some one is logon using your user id from any other computer. Multiple logons of users must be set to 1 in sap production systems. This can be allowed in DEV systems. But in production it must be restricted.
Go to rz10 and change the paramenter
login/multi_login_users. By default the value is 0 which is inactive. If you want to activate mupltiple logons for some users in sap like service users you have to change the parameter.
login/disable_multi_gui_login = you have to enter the user names in the values seperated by semi colon ; and dont leave spaces between the userids. Now you have to restart the instance.