code: func dedupDeltas(deltas Deltas) Deltas {
  	n := len(deltas)
  	if n < 2 {
  		return deltas
  	}
  	a := &deltas[n-1]
  	b := &deltas[n-2]
  	if out := isDup(a, b); out != nil {
  		d := append(Deltas{}, deltas[:n-2]...)
  		return append(d, *out)
  	}
  	return deltas
  }
docs: // re-listing and watching can deliver the same update multiple times in any
  // order. This will combine the most recent two deltas if they are the same.