Not that anyone has ever done this, but... If you revisit the Advent Calendar later in the same day in which you collected your prizes at 12:37am NST, you will get this slightly different message:
TNT Advent Calendar pop-up wrote:You already have the prize for this day!
... But in case you forgot, here's what you found! 1071 Neopoints!!!
Neovia Sewing Kit
Prigpants and Swolthy Snowglobe
Note the lack of three exclamation points on each item;)
Day 21: Christmas Aisha: Holiday Naughty List Snowie: Faeries Fortune Scratchcard (scratched for nothing, but I bought a Races for Riches and scratched it for 10k)
I've revisited the advent prizes before, too. Just because I couldn't remember if I'd done it or not!
Day 23: Christmas Aisha: Spiced Gingerbread Latte (2 days in a row?) Snowie: Battle Muffin
Because I cracked and got myself a 10-pack of stockings, I'm now getting the NC items for the rest of the month, too. Today's was a Christmas Rock in a Pet Bed, which is pretty cute.