Calvin and Hobbes by Bill Watterson for January 19, 1990
Mom: Calvin, you know you're not allowed to eat cookies before dinner! Put those away! Did you clean your room yet? Calvin: I'm not Calvin. I'm a duplicate. Mom: I don't want to hear about it. Now move! Ooh, some days that kid of mine... What are you doing in here?! Calvin: Why? Are you taking a survey?