message: docker: Use a set for adds to avoid duplicates.
diff: @@ -54,7 +54,7 @@ class DockerFile(drake.Node):
  for node in nodes:
  self.add(node, path)
  else:
  - self.__adds.setdefault(drake.Path(path), []).append(nodes)
  + self.__adds.setdefault(drake.Path(path), set()).add(nodes)
  def cmd(self, cmd):
  if isinstance(cmd, list):
  