You are correct in that you stated you can't do it for every member. I just assumed that it would be easy, sort of like me looking up a member on the list of members which takes no time at all, though I have no idea how many people asked.
When I registered on this site, I automatically received a PM welcoming me to the site. Could an automated confirmation not have been sent in the same manner when entering the contest?...or a pop-up stating that you have already entered the draw?
Anyway, it is what it is.....
Good luck to all those that entered!