Keynote: appear, disappear, reappear in Build Order
Unfortunately this is not possible to do. (Provide Kenyote feedback to Apple if you'd like to see this feature added!) Here are two possible alternatives:


Create 2 copies of the object on the slide and have them build in/out in sequence. This will keep your slide deck cleaner and make it easier to align the objects.

(objects shown separately for illustrative purposes; you'd want to align them perfectly!)  


As you suggested, create 2 copies of the slide with no transition (so the effect is seamless and there's no delay). This will make it easier to see how you've set up the effect in your list of slides. (But if nothing else on the slide changes, it might just be annoyingly redundant.)