I duplicated a file that had an existing story structure I’m using because I want to share it with others. I deleted a bunch of events and narrative folder items that are story specific but these still show up in the “add existing” popup and I don’t want that information shared (it’s just not relevant and causes confusion).
It may be that you deleted these events from your Narrative, but not from your whole timeline project (eg. they may still appear on Timeline View or Spreadsheet View). This would be why they are still appearing in this list, as they still exist in the project.
If you wish to delete an event from the entire project, then you can select it and choose Delete Item from the Item menu, or right click and Choose Delete Item.