Step 1: Create a world (superflat probably works best I think), load it up, walk around for a bit, then exit the world.

Step 2: Delete the world.

Step 3: Create another world with the same name as the deleted one (if using superflat, change the world preset to something besides the one the deleted world had).

Step 4: Volla! That should work!

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So, how did I do for my first blog/tutorial on Planet Minecraft?