angelina886 I'm sorry, I missed the tag that you were on a Mac. To kill a process, open Activity Monitor, inside Applications > Utilities, sort by name, double-click the name of the process, and you'll see the option to quit in the pop-up that appears.
For the Data folder, find the Sims 3.app, here by default:
Applications > EA Games
Right-click the .app (it has a green plumbob icon) and select Show Package Contents, then open:
Contents > Resources > The Sims 3.app > [right-click, Show Package Contents] > Contents > GameData > Shared > NonPackaged > Worlds
Delete SunsetValley.world inside, then repair the game and see whether you specifically get a duplicate copy of SV at the Main Menu.
If this helps, open the Sims 3 Packs folder, in the same location as the Sims 3.app, and open the folder for one of your EPs that's showing a duplicate world. For example, Moonlight Falls is Supernatural, which is EP07. Inside that, open GameData > Shared > NonPackaged > Worlds and delete the .world file inside, then repair again.
The EA App should repair the pack from which you deleted the .world file, but if it doesn't, you can delete the entire EP folder and download it again. Whether or not the repair restores the .world file, I'd like to know whether the duplicate world disappears from the Main Menu. The main world should disappear; this is a question of where the duplicate is coming from.