How to merge two ArrayLists without duplicates? [duplicate]

Either:

Set<Foo> fooSet = new LinkedHashSet<>(one);
fooSet.addAll(two);
List<Foo> finalFoo = new ArrayList<>(fooSet);

or

List<Foo> twoCopy = new ArrayList<>(two);
twoCopy.removeAll(one);
one.addAll(twoCopy);