UUID Crash

Open the launcher program, run the game, hit singleplayer then select world, get to Loading World Building Terrains then game crashes and brings back to launcher. Ran log and got: Keeping entity minecraft:pig that already exists with UUID d526d885-4e9f-4d19-8866-d4ef197bdfc2 in the debug message. Attempted to fix issue in MCedit but unable to run the plugin that fixes said issue.