Increasing Vengeance in hand or in your graveyard.
Dual Strike in hand or in exile.
You have an additional instant and/or sorcery card in hand.
({R} magic symbol) ({R} magic symbol) ({R} magic symbol) , plus an additional ({R} magic symbol) if Dual Strike was not foretold, plus enough mana to cast the additional instant or sorcery available.
Steps
Cast any other instant and/or sorcery spell by paying its mana cost.
Holding priority, cast Dual Strike by paying ({R} magic symbol) ({R} magic symbol) , copying the next instant or sorcery spell you cast with mana value 4 or less.
Cast Increasing Vengeance by paying ({R} magic symbol) ({R} magic symbol) , targeting the instant or sorcery spell cast in step 1.
Dual Strike triggers, copying Increasing Vengeance, with the copy targeting the original Increasing Vengeance.
Resolve the copy of Increasing Vengeance, copying Increasing Vengeance, with the copy targeting the original Increasing Vengeance.