Occurs in 1.3.1 and later.

I created a world to test out the barrier (haven't played new MC for a while, wanted to try out the new snapshots), and thought, whoops, meant to create a superflat world, not a default one. So, I quit, deleted the world, and made a new one (with the same name of 'New World'). The old world was also partly in this new one, causing there to be some default chunks, and mainly superflat chunks.

I'd have expected the game to stop the generation process - of course, the server simply doesn't quit until it's done generating - when I quit or delete the save, rather than it carrying on and causing it AND the new generation of the second world to both create the world.

This is presumably due to the fact that the server cannot be closed until AFTER world generation. Unfortunately, it just so happens that 2 servers end up running at the same time when you create a new one with the same name, leading to them both creating a different landscape that doesn't really blend together.

Of course, 1.3.1 is when SP first ran a server inside itself, so this doesn't occur on versions before that.

How to reproduce:
1. Create a world.
2. While it is still appearing, but the chunks around the player have already appeared, save and quit.
3. Delete the world.
4. Create a new world with the same name as the previous.
5. Some chunks will appear from the old generation, but most will be new (and when heading far outside, there will be no old chunks as the old server is stopped as soon as it's done initial generation - which just happens to be a little too slow).

I'd like to see the game pause before starting a new server if the old server is running, and then for it to start the server when the old server closes. (But only if they are both trying to access the same folder name.)
But only as a quick fix.
Later on, I'd prefer the server to stop and delete if it isn't generated when the game is quit - and for a message to be displayed on the client saying that it will delete the world.

(I'm surprised this hasn't been submitted already. And I know this has got to have been submitted and I won't have seen it, just because I just wrote that.)