Interesting Christmas card situation....

OK...here's an interesting Christmas card "situation".   You receive a Christmas card from someone; meanwhile, you have already sent them a card.  OK...they're off your list.  Right? Ahhh, but then a couple of weeks later, they send you another Christmas card!  Mmmm...strange.  They probably didn't realize it.

What to do? Do you send them another card?  Let them know they've sent you two cards?  Not say anything?