Duplicate Logins!!!

Hello List: I have tried but failed, since I am new to freeRADIUS, I humbly ask for help. I want to setup freeRADIUS so it does not allow more than one simultaneous login on the same UID/PW. I have read it somewhere that it can be done in freeRADIUS but not can't seem to find the file or the command string. Please help. Kirti