duplicate timelines need to be marked changed

if not, we might lose the duplicated timeline when not saved and trying to reload

x-pack/plugins/security_solution/public/timelines/components/open_timeline/use_update_timeline.tsx

@@ -53,7 +53,8 @@ export const useUpdateTimeline = () => {
     }: UpdateTimeline) => {
       let _timeline = timeline;
       if (duplicate) {
-        _timeline = { ...timeline, updated: undefined, changed: undefined, version: null };
+        // `changed: true` in this case indicates that the timeline hasn't been saved yet
+        _timeline = { ...timeline, updated: undefined, changed: true, version: null };
       }
       if (!isEmpty(_timeline.indexNames)) {
         dispatch(