gistfile1.scala

  def foreachUntilDup[A, U](c: Seq[A])(f: A => U) {
    @tailrec
    def traverse(seen: Set[A], remaining: Seq[A]) {
      remaining.headOption match {
        case Some(v) if !seen.contains(v) =>
          f(v)
          traverse(seen + v, remaining.tail)
        case _ => // done
      }
    }
    traverse(Set.empty, c)
  }
  
// Example:
// foreachUntilDup(Seq(1,2,3,3,4))(println)
// 1
// 2
// 3

looks great