My oldest daughter just turned four. In preparation for her birthday party, we instructed her that if someone gave her a gift containing something she already owns, she should simply say thank you (and not point out that she already owns it).
At the party, someone gave her a few books and she already had all but one of them. She began to speak but caught herself, then I could see the little gears in her head turning. She then said, “Mommy, I don’t have this one!”
She followed our instructions to the best of her ability, and it was hilarious.