What happens next is that Calvin and his future self decide to go back in time halfway and force THAT Calvin to do it. Of course, the Calvin of that time doesn't want to do either; the three of them all end up fighting each other, while Hobbes ends up writing the story that Calvin was supposed to do for homework. Calvin then turns it in without reading it... he gets an A, but he's mad at Hobbes anyway because, when he gets it back, Calvin realizes that the story (which was about Calvin trying to use his time machine to get out of writing a story for homework) made him look like an idiot.
Best line in the story (during the fight among Calvins): go ahead and hit me, it'll be the future Calvin who hurts!
An episode of the Noddy animated series has the following plot.
Noddy needs to go pick up Martha Monkey at the station. But it's such a nice, sunny day that he would prefer to play around outside. He gets an idea to solve this dilemma. He casts a duplication spell on himself and his car and tells the duplicate to go fetch Martha while he goes out to play. Later, Noddy is out having fun when he suddenly spots his duplicate. It turns out that the duplicate also preferred playing outside to doing the errand so he also cast a duplication spell. Then they see another duplicate, and another...
I think this story makes for a nice simple illustration of one of our perennial decision theoretic issues: When making decisions you should take into account that agents identical to yourself will make the same decision in the same situation. A common real-life example of the Noddy problem is when we try to pawn off our dietary problems to our future selves.