I created a world in "amplified" and then deleted it. Immediately after that, I created a normal world in survival mode with the exact same name. Chunks werre a bizarre mixture of plains and extreme hills biomes.

The development console gave me the following error: [Server thread/ERROR]: Couldn't save chunk; already in use by another instance of Minecraft?

I recreated the world (using re-create) and the world generated properly.

I was able to duplicate this on subsequent worlds by first creating a world in amplified, deleting it through the launcher, and immediately creating a normal world with the exact same name.