How does a duel work in the game BANG?
A duel is a special attack that must be responded to by discarding a bang. If the person the duel is played on is unable to discard a bang they take a hit. If they are able to it goes back to the person who played the duel. If they can't discard a bang they take a hit however if they can the process repeats by going back and forth between the players until one of them is no longer able to discard a bang and that player takes a hit.

Duel


  With this card you can challenge any other
  player (staring him in the eyes!), regardless
  of the distance. The challenged player may discard a BANG!
  card (even though it is not his turn!). If he does, you may
  discard a BANG! card, and so on: the first player failing to
  discard a BANG! card loses one life point, and the duel is
  over. You cannot play Missed! or use the Barrel during a
  duel. The Duel is not a BANG! card. BANG! cards discarded
  during a Duel are not accounted towards the “one BANG!
  card” limitation.