Yes, that's correct, assuming the original playing was for the normal effect of the Duration. The rules try to be simple and not consider to many "corner cases".
The rule is that if a card (like Throne Room or Scepter) plays a Duration card extra times, it stays out as long as the Duration card. In your case, the Scepter did play a Duration card an extrav time, so the Scepter stays out as long as the Duration card. Since a Duration card stays out generally for at least one turn when played "normally", so will the Scepter.