On the server (pseudo-code, use whatever server-side language you like, optimize…):
Code:
if the session does not exist, then
open the session,
put a opening-timestamp in there,
send the notice with a link to the requested URL;
else
if the timestamp in the session is less than 20 seconds old, then
send the notice with a link to the requested URL;
else
serve the requested URL.
It would be good for the notice to indicate the number of seconds to wait before the user shall click the link.
Yves.