How to get list of all duplicates in two collections

Duplicates.java

public class Duplicates {
    private static <T> BiFunction<Processing<T>, T, Processing<T>> getReducer(){
        return (acc, elem) -> {
            if (acc.uniques.contains(elem)) {
                acc.dupes.add(elem);
            } else {
                acc.uniques.add(elem);
            }
            return acc;
        };
    }

    private static <T> BinaryOperator<Processing<T>> getCombiner(){
        return (o1, o2) -> {
            final Set<T> newDupes = new HashSet<>(o1.uniques);
            newDupes.retainAll(o2.uniques);
            newDupes.addAll(o2.dupes);
            o1.dupes.addAll(newDupes);
            return o1;
        };
    }

    public static <T> Set<T> getDuplicates(final Collection<T> elements){
        return elements.stream().reduce(
                new Processing<>(),
                getReducer(),
                getCombiner()
        ).dupes;
    }

    private static class Processing<T>{
        public final Set<T> uniques = new HashSet<>();
        public final Set<T> dupes = new HashSet<>();
    }
}

/* ------------------------------------------ */

public class AnyDuplicate {
    private static <T> BiFunction<Processing<T>, T, Processing<T>> getReducer() {
        return (acc, elem) -> {
            if (acc.dupe == null) {
                if (acc.uniques.contains(elem)) {
                    acc.dupe = elem;
                } else {
                    acc.uniques.add(elem);
                }
            }
            return acc;
        };
    }

    private static <T> BinaryOperator<Processing<T>> getCombiner() {
        return (o1, o2) -> {
            T dupe = o1.dupe;
            if (dupe == null) {
                dupe = o2.dupe;
            }
            if (dupe == null) {
                dupe = o2.uniques.stream()
                        .filter(o1.uniques::contains)
                        .findAny()
                        .orElse(null);
            }
            if(dupe == null){
                o1.uniques.addAll(o2.uniques);
            } else {
                o1.dupe = dupe;
            }
            return o1;
        };
    }

    public static <T> T getAnyDuplicate(final Collection<T> elements) {
        return elements.stream().reduce(
                new Processing<>(),
                getReducer(),
                getCombiner()
        ).dupe;
    }

    private static class Processing<T> {
        public final Set<T> uniques = new HashSet<>();
        public T dupe;
    }
}