Already existing works that fulfill the prompt can't count for a new prompt unless you continue that story for the new prompt. So, for example, say N37 is called on Day Two. You write your Frodo story where he lives. On Day 29, G50 is called. You can continue or add to the story you've already started for credit; i.e., you might write an epilogue about how Frodo is getting on two weeks later. Or you might add an extra hypothermia-related scene into the existing story.

Pretty much, to count for credit, you should create something new for the day's prompt, whether adding to an existing work or starting something from scratch. :)