如何在python3中删除列表中的重复元素？

环境：python 3.6.4

我有两个列表，
list1是嵌套的单词列表，就像

[['this', 'is', 'a', 'pen', 'that', 'is', 'a', 'desk'],
 ['this', 'is', 'an', 'apple']]

list2是要从list1中删除的单词列表，例如

['a', 'an']

我想获得新的清单，例如

[['this', 'is', 'pen', 'that', 'is', 'desk'],
 ['this', 'is', 'apple']]

并且不会更改list1。

我在下面的代码中编写了代码，但是我的代码破坏了list1，我的代码哪里错了？

def remove_duplicate_element_in_nested_list(li1, li2):
    """
    :param li1: <list> nested_sentences
    :param li2: <list> words_to_remove
    :return: <list>
    """
    ret = []
    for el1 in li1:
        ret.append(el1)

    for i in range(len(ret)):
        for el2 in li2:
            try:
                # list.remove() remove only one element. so loop this.
                for el in ret[i]:
                    ret[i].remove(el2)
            except ValueError:
                None

    return ret

words = [['this', 'is', 'a', 'pen', 'this', 'is', 'a', 'desk'], ['this', 'is', 'an', 'apple']]
stop_words = ['a', 'an']

print(words)
# shows [['this', 'is', 'a', 'pen', 'that', 'is', 'a', 'desk'], ['this', 'is', 'an', 'apple']]
new_words = remove_duplicate_element_in_nested_list(words, stop_words)
print(words)
# shows [['this', 'is', 'pen', 'that', 'is', 'desk'], ['this', 'is', 'apple']]