How to build a new list containing all entries from existing list AND a copy of each entry with one field modified?

I have a list of some Object called list. Using list.stream(), I need to create a new list of the same Object where the new list contains all of the original entries AND the new list contains a copy of each entry with one field modified. I know how to do this using list.stream() twice but I'd like to do it under one stream.

This is how I've accomplished the task using list.stream() twice

newList = list.stream().collect(Collectors.toList());
newList.addAll(list.stream().map(l -> {SomeObject a = new SomeObject(l);
                      a.setField1("New Value");
                      return a;
                      }).collect(Collectors.toList())
                );

Use flatMap (assuming you don't mind the original and derived values being interleaved):

newList = list.stream()
    .flatMap(l -> {
      SomeObject a = new SomeObject(l);
      a.setField1("New Value");
      return Stream.of(l, a);
    })
    .collect(toList());

If it's just that you don't want to use stream() twice, you could avoid the first one using addAll; and the unnecessary collect for the second:

newList = new ArrayList<>(list.size() * 2);
newList.addAll(list);
list.stream()
    .map(l -> {
        SomeObject a = new SomeObject(l);
        a.setField1("New Value");
        return a;
    })
    .forEach(newList::add);