What do you say when someone gives you a present: "thank you". What do you say when it's a present you already have?: You simply say, "Thank you". What if you hate the present? The answer again is "thank you".
To my horror, when my sister flew in from South Carolina carrying a gift she thoughtful picked out, things did not go well. My son tore the paper open and exclaimed, "But I already have that guy"! All of my hard work, down the drain. I pulled him aside and reviewed what I tried to teach him and frankly I don't know where I went wrong. Maybe he needs to get just a little bigger.
I did find a great website to help train our little ones on the proper behavior for a birthday boy or girl. It's not that I want him to act like a robot, but I desperately want to avoid raising a spoiled child who is chasing down presents from everyone who enters the home!
One of the first pieces of advice is to pass out invitations in school to all or none of the class. I know this one is tricky, but I agree with it whole heartedly. If you're wondering, they offer the same advice I tried with my son: instructing kids to thank each guest for the gift even if they don't like it.
Hopefully by college, he'll get the idea!