This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Eric is exactly right. Even if you have client-side guards to prevent this, you still need code not he server to detect and prevent this condition as you can never be certain that the client-side guards will be used.