J.
I would let it go. Regift or donate it, unless it's some sort of really expensive designer thing worth returning. My kids received several gifts this year that they already have or don't need - it just hurts the giver's feelings, I think, to be asked for a receipt if they didn't have one to include with the package. (if I have a receipt, I always include it, but sometimes I don't have it and then I'd feel obligated to buy a new gift, and honestly - not worth the aggravation for anyone involved.)