If you have a world named 'x', and then create a new world with the name of 'x.' (put a dot after the world name), instead of creating a new world, it loads the world with the name 'x', no matter what gamemode and seed you selected. As you can see, the world Skyscrapercity reamains on the same place.

Example: I created a worl named 'Weird Glitch'. Then, I created a new world named 'Weird Glitch.'. Instead of creating a new world, named 'Weird Glitch.', it just loads the 'Weird Glitch' world (pictures included).