D-Link DFL-1600 User Manual page 163

17.4. Scenarios: User Authentication Configuration 143
Allow multiple logins per username– If this one is selected,
the firewall will allow users from different source IP addresses,
but with the same username, to be simultaneous logged on.
Allow one login per username, disallow the rest– If this is
selected, the firewall will only allow one simultaneous username
to be logged on. That is, if a user from another IP address tries
to authenticate with the same username as that of an already
authenticated user, the firewall will disallow this login.
Allow one login per username– If this is selected, the firewall
will only allow one simultaneous username to be logged on. If a
user from another IP address tries to authenticate with the same
username as that of an already authenticated user, the firewall
will check if the authenticated user has been idle for a period of
time. If so, the old user will be removed, and this new user will
be logged in. If not, the new login-request will be rejected. The
time period for this option can be defined in the edit box.
D-Link Firewalls User's Guide