Allows multiple duplicate keys in a dict. Not a complete implementation (e.g. __getitem__ missing, etc.) and not the most efficient one; just a proof of concept.

duplicate_dict.py

class DuplicateDict(object):
    def __init__(self):
        self._items = []

    def items(self):
        return self._items

    def __iter__(self):
        def wrap():
            for key, value in self._items:
                yield key
        return wrap()

    def get(self, key, default):
        # O(n)
        for k, v in self._items:
            if k == key:
                return value
        return default

    def add(self, key, value):
        self._items.append((key, value))