Map<Character, Character> map=new HashMap<Character, Character>();
        map.put('a', 'a');
        map.put('b', 'a');
        map.put('c', 'b');
        List<Character> list=new ArrayList<Character>();
        list.add('a');
        list.add('a');
        list.add('b');
        Iterator<Entry<Character, Character>> iter = map.entrySet().iterator();
        Map.Entry entry;
        while (iter.hasNext()) {
            entry =iter.next();
            if (Collections.frequency(list, entry.getValue())>=2) {
                System.out.println(entry.getKey()+":"+entry.getValue());

            }   
        }