One solution is to use a Map, that stores a List as value. When adding a new KV-pair, check if the key exists. If it does NOT exist, create a new List, add the value to the List and add it to the Map. If the key DOES exist, get the List and add the value to the List:

        if ( ! map.containsKey( key ) ) {
            List list = new ArrayList( );
            list.add( value);
            map.put( key, list);
        }
        else {
            List list = (List) map.get(key);
            list.add( value );
        }

Or if you want to have a single list.add():

        if ( ! map.containsKey( key ) ) {
            List list = new ArrayList( );
            map.put( key, list);
        }
        
        List list = (List) map.get(key);
        list.add( value );