This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
One way would be to remember what was your last result from random, and if it matches your newly picked number, just keep picking them till they don't match. Better solutions would include some kind of a Collection, ie. a Set. Once you draw your number you remove it, next time you pick from what is left in your bag.