You definitely don't need to duplicate any content. The beauty of Construct is that everything may be reusable - objects, layers, events.
Two levels in the game can share many resources. For example, layers like background or HUD can be set as Global. This way you only need to create it once on the first layout.
Add instances of objects, don't clone them! If you have a zombie on level 1 and want to add a zombie on level 2, just drag&drop the same object, don't create Zombie2 sprite.
Also, set the same event sheet for both layouts. If any events need to be different on level 2 for example, you can add specific conditions, for example "If LevelNumber=2 ....."