In the DefaultLoginModule, it supports to set "singleLogin" option parameter which disallows the same user to login to use the portal at the same time from different browsers/machines
Unfortunately it doesn't work properly. I can not login with the same user after logged-out.
Moreover, the GateIn documentation is also missing this part. I think we could add this information in the section https://docs.jboss.org/author/display/GTNPORTAL36/Authentication+and+Authorization+intro